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This chapter provides a tutorial introduction to MySQL by showing
how to use the mysql client program to create and
use a simple database. mysql (sometimes referred
to as the “terminal monitor” or just
“monitor”) is an interactive program that allows you to
connect to a MySQL server, run queries, and view the results.
mysql may also be used in batch mode: you place
your queries in a file beforehand, then tell
mysql to execute the contents of the file. Both
ways of using mysql are covered here.
To see a list of options provided by mysql,
invoke it with the --help option:
shell> mysql --help
This chapter assumes that mysql is installed on
your machine and that a MySQL server is available to which you can
connect. If this is not true, contact your MySQL administrator. (If
you are the administrator, you need to consult
the relevant portions of this manual, such as
Chapter 5, MySQL Server Administration.)
This chapter describes the entire process of setting up and using a
database. If you are interested only in accessing an existing
database, you may want to skip over the sections that describe how
to create the database and the tables it contains.
Because this chapter is tutorial in nature, many details are
necessarily omitted. Consult the relevant sections of the manual for
more information on the topics covered here.
3.1. Connecting to and Disconnecting from the Server
To connect to the server, you will usually need to provide a MySQL
user name when you invoke mysql and, most
likely, a password. If the server runs on a machine other than the
one where you log in, you will also need to specify a host name.
Contact your administrator to find out what connection parameters
you should use to connect (that is, what host, user name, and
password to use). Once you know the proper parameters, you should
be able to connect like this:
shell> mysql -h host -u user -p
Enter password: ********
host and user represent the
host name where your MySQL server is running and the user name of
your MySQL account. Substitute appropriate values for your setup.
The ******** represents your password; enter it
when mysql displays the Enter
password: prompt.
If that works, you should see some introductory information
followed by a mysql> prompt:
shell> mysql -h host -u user -p
Enter password: ********
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 25338 to server version: 5.0.91-standard
Type 'help;' or '\h' for help. Type '\c' to clear the buffer.
mysql>
The mysql> prompt tells you that
mysql is ready for you to enter commands.
If you are logging in on the same machine that MySQL is running
on, you can omit the host, and simply use the following:
shell> mysql -u user -p
If, when you attempt to log in, you get an error message such as
ERROR 2002 (HY000): Can't connect to local MySQL server
through socket '/tmp/mysql.sock' (2), it means that
the MySQL server daemon (Unix) or service (Windows) is not
running. Consult the administrator or see the section of
Chapter 2, Installing and Upgrading MySQL that is appropriate to your operating
system.
For help with other problems often encountered when trying to log
in, see Section B.5.2, “Common Errors When Using MySQL Programs”.
Some MySQL installations allow users to connect as the anonymous
(unnamed) user to the server running on the local host. If this is
the case on your machine, you should be able to connect to that
server by invoking mysql without any options:
shell> mysql
After you have connected successfully, you can disconnect any time
by typing QUIT (or \q) at
the mysql> prompt:
mysql> QUIT
Bye
On Unix, you can also disconnect by pressing Control-D.
Most examples in the following sections assume that you are
connected to the server. They indicate this by the
mysql> prompt.
Make sure that you are connected to the server, as discussed in
the previous section. Doing so does not in itself select any
database to work with, but that is okay. At this point, it is more
important to find out a little about how to issue queries than to
jump right in creating tables, loading data into them, and
retrieving data from them. This section describes the basic
principles of entering commands, using several queries you can try
out to familiarize yourself with how mysql
works.
Here is a simple command that asks the server to tell you its
version number and the current date. Type it in as shown here
following the mysql> prompt and press Enter:
mysql> SELECT VERSION(), CURRENT_DATE;
+----------------+--------------+
| VERSION() | CURRENT_DATE |
+----------------+--------------+
| 5.0.7-beta-Max | 2005-07-11 |
+----------------+--------------+
1 row in set (0.01 sec)
mysql>
This query illustrates several things about
mysql:
A command normally consists of an SQL statement followed by a
semicolon. (There are some exceptions where a semicolon may be
omitted. QUIT, mentioned earlier, is one of
them. We'll get to others later.)
When you issue a command, mysql sends it to
the server for execution and displays the results, then prints
another mysql> prompt to indicate that
it is ready for another command.
mysql displays query output in tabular form
(rows and columns). The first row contains labels for the
columns. The rows following are the query results. Normally,
column labels are the names of the columns you fetch from
database tables. If you're retrieving the value of an
expression rather than a table column (as in the example just
shown), mysql labels the column using the
expression itself.
mysql shows how many rows were returned and
how long the query took to execute, which gives you a rough
idea of server performance. These values are imprecise because
they represent wall clock time (not CPU or machine time), and
because they are affected by factors such as server load and
network latency. (For brevity, the “rows in set”
line is sometimes not shown in the remaining examples in this
chapter.)
Keywords may be entered in any lettercase. The following queries
are equivalent:
mysql> SELECT VERSION(), CURRENT_DATE;
mysql> select version(), current_date;
mysql> SeLeCt vErSiOn(), current_DATE;
Here is another query. It demonstrates that you can use
mysql as a simple calculator:
mysql> SELECT SIN(PI()/4), (4+1)*5;
+------------------+---------+
| SIN(PI()/4) | (4+1)*5 |
+------------------+---------+
| 0.70710678118655 | 25 |
+------------------+---------+
1 row in set (0.02 sec)
The queries shown thus far have been relatively short, single-line
statements. You can even enter multiple statements on a single
line. Just end each one with a semicolon:
mysql> SELECT VERSION(); SELECT NOW();
+----------------+
| VERSION() |
+----------------+
| 5.0.7-beta-Max |
+----------------+
1 row in set (0.00 sec)
+---------------------+
| NOW() |
+---------------------+
| 2005-07-11 17:59:36 |
+---------------------+
1 row in set (0.00 sec)
A command need not be given all on a single line, so lengthy
commands that require several lines are not a problem.
mysql determines where your statement ends by
looking for the terminating semicolon, not by looking for the end
of the input line. (In other words, mysql
accepts free-format input: it collects input lines but does not
execute them until it sees the semicolon.)
Here is a simple multiple-line statement:
mysql> SELECT
-> USER()
-> ,
-> CURRENT_DATE;
+---------------+--------------+
| USER() | CURRENT_DATE |
+---------------+--------------+
| jon@localhost | 2005-07-11 |
+---------------+--------------+
In this example, notice how the prompt changes from
mysql> to -> after you
enter the first line of a multiple-line query. This is how
mysql indicates that it has not yet seen a
complete statement and is waiting for the rest. The prompt is your
friend, because it provides valuable feedback. If you use that
feedback, you can always be aware of what mysql
is waiting for.
If you decide you do not want to execute a command that you are in
the process of entering, cancel it by typing
\c:
mysql> SELECT
-> USER()
-> \c
mysql>
Here, too, notice the prompt. It switches back to
mysql> after you type \c,
providing feedback to indicate that mysql is
ready for a new command.
The following table shows each of the prompts you may see and
summarizes what they mean about the state that
mysql is in.
In the MySQL 5.0 series, the /*> prompt was
implemented in MySQL 5.0.6.
Multiple-line statements commonly occur by accident when you
intend to issue a command on a single line, but forget the
terminating semicolon. In this case, mysql
waits for more input:
mysql> SELECT USER()
->
If this happens to you (you think you've entered a statement but
the only response is a -> prompt), most
likely mysql is waiting for the semicolon. If
you don't notice what the prompt is telling you, you might sit
there for a while before realizing what you need to do. Enter a
semicolon to complete the statement, and mysql
executes it:
mysql> SELECT USER()
-> ;
+---------------+
| USER() |
+---------------+
| jon@localhost |
+---------------+
The '> and "> prompts
occur during string collection (another way of saying that MySQL
is waiting for completion of a string). In MySQL, you can write
strings surrounded by either “'”
or “"” characters (for example,
'hello' or "goodbye"), and
mysql lets you enter strings that span multiple
lines. When you see a '> or
"> prompt, it means that you have entered a
line containing a string that begins with a
“'” or
“"” quote character, but have not
yet entered the matching quote that terminates the string. This
often indicates that you have inadvertently left out a quote
character. For example:
mysql> SELECT * FROM my_table WHERE name = 'Smith AND age < 30;
'>
If you enter this SELECT statement,
then press Enter and wait for the result, nothing
happens. Instead of wondering why this query takes so long, notice
the clue provided by the '> prompt. It tells
you that mysql expects to see the rest of an
unterminated string. (Do you see the error in the statement? The
string 'Smith is missing the second single
quote mark.)
At this point, what do you do? The simplest thing is to cancel the
command. However, you cannot just type \c in
this case, because mysql interprets it as part
of the string that it is collecting. Instead, enter the closing
quote character (so mysql knows you've finished
the string), then type \c:
mysql> SELECT * FROM my_table WHERE name = 'Smith AND age < 30;
'> '\c
mysql>
The prompt changes back to mysql>,
indicating that mysql is ready for a new
command.
The `> prompt is similar to the
'> and "> prompts, but
indicates that you have begun but not completed a backtick-quoted
identifier.
It is important to know what the '>,
">, and `> prompts
signify, because if you mistakenly enter an unterminated string,
any further lines you type appear to be ignored by
mysql — including a line containing
QUIT. This can be quite confusing, especially
if you do not know that you need to supply the terminating quote
before you can cancel the current command.
3.3. Creating and Using a Database
Once you know how to enter commands, you are ready to access a
database.
Suppose that you have several pets in your home (your menagerie)
and you would like to keep track of various types of information
about them. You can do so by creating tables to hold your data and
loading them with the desired information. Then you can answer
different sorts of questions about your animals by retrieving data
from the tables. This section shows you how to perform the
following operations:
The menagerie database is simple (deliberately), but it is not
difficult to think of real-world situations in which a similar
type of database might be used. For example, a database like this
could be used by a farmer to keep track of livestock, or by a
veterinarian to keep track of patient records. A menagerie
distribution containing some of the queries and sample data used
in the following sections can be obtained from the MySQL Web site.
It is available in both compressed tar file and
Zip formats at http://dev.mysql.com/doc/.
Use the SHOW statement to find out
what databases currently exist on the server:
mysql> SHOW DATABASES;
+----------+
| Database |
+----------+
| mysql |
| test |
| tmp |
+----------+
The mysql database describes user access
privileges. The test database often is
available as a workspace for users to try things out.
The list of databases displayed by the statement may be different
on your machine; SHOW DATABASES
does not show databases that you have no privileges for if you do
not have the SHOW DATABASES
privilege. See Section 12.5.5.11, “SHOW DATABASES Syntax”.
If the test database exists, try to access it:
mysql> USE test
Database changed
USE, like QUIT,
does not require a semicolon. (You can terminate such statements
with a semicolon if you like; it does no harm.) The
USE statement is special in another
way, too: it must be given on a single line.
You can use the test database (if you have
access to it) for the examples that follow, but anything you
create in that database can be removed by anyone else with access
to it. For this reason, you should probably ask your MySQL
administrator for permission to use a database of your own.
Suppose that you want to call yours menagerie.
The administrator needs to execute a command like this:
mysql> GRANT ALL ON menagerie.* TO 'your_mysql_name'@'your_client_host';
where your_mysql_name is the MySQL user name
assigned to you and your_client_host is the
host from which you connect to the server.
3.3.1. Creating and Selecting a Database
If the administrator creates your database for you when setting
up your permissions, you can begin using it. Otherwise, you need
to create it yourself:
mysql> CREATE DATABASE menagerie;
Under Unix, database names are case sensitive (unlike SQL
keywords), so you must always refer to your database as
menagerie, not as
Menagerie, MENAGERIE, or
some other variant. This is also true for table names. (Under
Windows, this restriction does not apply, although you must
refer to databases and tables using the same lettercase
throughout a given query. However, for a variety of reasons, the
recommended best practice is always to use the same lettercase
that was used when the database was created.)
Note
If you get an error such as ERROR 1044 (42000):
Access denied for user 'monty'@'localhost' to database
'menagerie' when attempting to create a database,
this means that your user account does not have the necessary
privileges to do so. Discuss this with the administrator or
see Section 5.4, “The MySQL Access Privilege System”.
Creating a database does not select it for use; you must do that
explicitly. To make menagerie the current
database, use this command:
mysql> USE menagerie
Database changed
Your database needs to be created only once, but you must select
it for use each time you begin a mysql
session. You can do this by issuing a
USE statement as shown in the
example. Alternatively, you can select the database on the
command line when you invoke mysql. Just
specify its name after any connection parameters that you might
need to provide. For example:
shell> mysql -h host -u user -p menagerie
Enter password: ********
Important
menagerie in the command just shown is
not your password. If you
want to supply your password on the command line after the
-p option, you must do so with no
intervening space (for example, as
-pmypassword, not as -p
mypassword). However, putting your password on the
command line is not recommended, because doing so exposes it
to snooping by other users logged in on your machine.
Note
You can see at any time which database is currently selected
using SELECT
DATABASE().
Creating the database is the easy part, but at this point it is
empty, as SHOW TABLES tells you:
mysql> SHOW TABLES;
Empty set (0.00 sec)
The harder part is deciding what the structure of your database
should be: what tables you need and what columns should be in
each of them.
You want a table that contains a record for each of your pets.
This can be called the pet table, and it
should contain, as a bare minimum, each animal's name. Because
the name by itself is not very interesting, the table should
contain other information. For example, if more than one person
in your family keeps pets, you might want to list each animal's
owner. You might also want to record some basic descriptive
information such as species and sex.
How about age? That might be of interest, but it is not a good
thing to store in a database. Age changes as time passes, which
means you'd have to update your records often. Instead, it is
better to store a fixed value such as date of birth. Then,
whenever you need age, you can calculate it as the difference
between the current date and the birth date. MySQL provides
functions for doing date arithmetic, so this is not difficult.
Storing birth date rather than age has other advantages, too:
You can use the database for tasks such as generating
reminders for upcoming pet birthdays. (If you think this
type of query is somewhat silly, note that it is the same
question you might ask in the context of a business database
to identify clients to whom you need to send out birthday
greetings in the current week or month, for that
computer-assisted personal touch.)
You can calculate age in relation to dates other than the
current date. For example, if you store death date in the
database, you can easily calculate how old a pet was when it
died.
You can probably think of other types of information that would
be useful in the pet table, but the ones
identified so far are sufficient: name, owner, species, sex,
birth, and death.
Use a CREATE TABLE statement to
specify the layout of your table:
mysql> CREATE TABLE pet (name VARCHAR(20), owner VARCHAR(20),
-> species VARCHAR(20), sex CHAR(1), birth DATE, death DATE);
VARCHAR is a good choice for the
name, owner, and
species columns because the column values
vary in length. The lengths in those column definitions need not
all be the same, and need not be 20. You can
normally pick any length from 1 to
65535, whatever seems most reasonable to you.
Note
Prior to MySQL 5.0.3, the upper limit was 255.) If you make a
poor choice and it turns out later that you need a longer
field, MySQL provides an ALTER
TABLE statement.
Several types of values can be chosen to represent sex in animal
records, such as 'm' and
'f', or perhaps 'male' and
'female'. It is simplest to use the single
characters 'm' and 'f'.
The use of the DATE data type for
the birth and death
columns is a fairly obvious choice.
Once you have created a table, SHOW
TABLES should produce some output:
mysql> SHOW TABLES;
+---------------------+
| Tables in menagerie |
+---------------------+
| pet |
+---------------------+
To verify that your table was created the way you expected, use
a DESCRIBE statement:
mysql> DESCRIBE pet;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name | varchar(20) | YES | | NULL | |
| owner | varchar(20) | YES | | NULL | |
| species | varchar(20) | YES | | NULL | |
| sex | char(1) | YES | | NULL | |
| birth | date | YES | | NULL | |
| death | date | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+
You can use DESCRIBE any time,
for example, if you forget the names of the columns in your
table or what types they have.
For more information about MySQL data types, see
Chapter 10, Data Types.
3.3.3. Loading Data into a Table
After creating your table, you need to populate it. The
LOAD DATA and
INSERT statements are useful for
this.
Suppose that your pet records can be described as shown here.
(Observe that MySQL expects dates in
'YYYY-MM-DD' format; this may be different
from what you are used to.)
Because you are beginning with an empty table, an easy way to
populate it is to create a text file containing a row for each
of your animals, then load the contents of the file into the
table with a single statement.
You could create a text file pet.txt
containing one record per line, with values separated by tabs,
and given in the order in which the columns were listed in the
CREATE TABLE statement. For
missing values (such as unknown sexes or death dates for animals
that are still living), you can use NULL
values. To represent these in your text file, use
\N (backslash, capital-N). For example, the
record for Whistler the bird would look like this (where the
whitespace between values is a single tab character):
Whistler Gwen bird \N 1997-12-09 \N
To load the text file pet.txt into the
pet table, use this statement:
mysql> LOAD DATA LOCAL INFILE '/path/pet.txt' INTO TABLE pet;
If you created the file on Windows with an editor that uses
\r\n as a line terminator, you should use
this statement instead:
mysql> LOAD DATA LOCAL INFILE '/path/pet.txt' INTO TABLE pet
-> LINES TERMINATED BY '\r\n';
(On an Apple machine running OS X, you would likely want to use
LINES TERMINATED BY '\r'.)
You can specify the column value separator and end of line
marker explicitly in the LOAD
DATA statement if you wish, but the defaults are tab
and linefeed. These are sufficient for the statement to read the
file pet.txt properly.
If the statement fails, it is likely that your MySQL
installation does not have local file capability enabled by
default. See Section 5.3.4, “Security Issues with LOAD
DATA LOCAL”, for information
on how to change this.
When you want to add new records one at a time, the
INSERT statement is useful. In
its simplest form, you supply values for each column, in the
order in which the columns were listed in the
CREATE TABLE statement. Suppose
that Diane gets a new hamster named “Puffball.” You
could add a new record using an
INSERT statement like this:
mysql> INSERT INTO pet
-> VALUES ('Puffball','Diane','hamster','f','1999-03-30',NULL);
String and date values are specified as quoted strings here.
Also, with INSERT, you can insert
NULL directly to represent a missing value.
You do not use \N like you do with
LOAD DATA.
From this example, you should be able to see that there would be
a lot more typing involved to load your records initially using
several INSERT statements rather
than a single LOAD DATA
statement.
3.3.4. Retrieving Information from a Table
The SELECT statement is used to
pull information from a table. The general form of the statement
is:
SELECT what_to_select
FROM which_table
WHERE conditions_to_satisfy;
what_to_select indicates what you
want to see. This can be a list of columns, or
* to indicate “all columns.”
which_table indicates the table from
which you want to retrieve data. The WHERE
clause is optional. If it is present,
conditions_to_satisfy specifies one
or more conditions that rows must satisfy to qualify for
retrieval.
3.3.4.1. Selecting All Data
The simplest form of SELECT
retrieves everything from a table:
mysql> SELECT * FROM pet;
+----------+--------+---------+------+------------+------------+
| name | owner | species | sex | birth | death |
+----------+--------+---------+------+------------+------------+
| Fluffy | Harold | cat | f | 1993-02-04 | NULL |
| Claws | Gwen | cat | m | 1994-03-17 | NULL |
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
| Fang | Benny | dog | m | 1990-08-27 | NULL |
| Bowser | Diane | dog | m | 1979-08-31 | 1995-07-29 |
| Chirpy | Gwen | bird | f | 1998-09-11 | NULL |
| Whistler | Gwen | bird | NULL | 1997-12-09 | NULL |
| Slim | Benny | snake | m | 1996-04-29 | NULL |
| Puffball | Diane | hamster | f | 1999-03-30 | NULL |
+----------+--------+---------+------+------------+------------+
This form of SELECT is useful
if you want to review your entire table, for example, after
you've just loaded it with your initial data set. For example,
you may happen to think that the birth date for Bowser doesn't
seem quite right. Consulting your original pedigree papers,
you find that the correct birth year should be 1989, not 1979.
There are at least two ways to fix this:
Edit the file pet.txt to correct the
error, then empty the table and reload it using
DELETE and
LOAD DATA:
mysql> DELETE FROM pet;
mysql> LOAD DATA LOCAL INFILE 'pet.txt' INTO TABLE pet;
However, if you do this, you must also re-enter the record
for Puffball.
Fix only the erroneous record with an
UPDATE statement:
mysql> UPDATE pet SET birth = '1989-08-31' WHERE name = 'Bowser';
The UPDATE changes only the
record in question and does not require you to reload the
table.
3.3.4.2. Selecting Particular Rows
As shown in the preceding section, it is easy to retrieve an
entire table. Just omit the WHERE clause
from the SELECT statement. But
typically you don't want to see the entire table, particularly
when it becomes large. Instead, you're usually more interested
in answering a particular question, in which case you specify
some constraints on the information you want. Let's look at
some selection queries in terms of questions about your pets
that they answer.
You can select only particular rows from your table. For
example, if you want to verify the change that you made to
Bowser's birth date, select Bowser's record like this:
mysql> SELECT * FROM pet WHERE name = 'Bowser';
+--------+-------+---------+------+------------+------------+
| name | owner | species | sex | birth | death |
+--------+-------+---------+------+------------+------------+
| Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 |
+--------+-------+---------+------+------------+------------+
The output confirms that the year is correctly recorded as
1989, not 1979.
String comparisons normally are case-insensitive, so you can
specify the name as 'bowser',
'BOWSER', and so forth. The query result is
the same.
You can specify conditions on any column, not just
name. For example, if you want to know
which animals were born during or after 1998, test the
birth column:
mysql> SELECT * FROM pet WHERE birth >= '1998-1-1';
+----------+-------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+----------+-------+---------+------+------------+-------+
| Chirpy | Gwen | bird | f | 1998-09-11 | NULL |
| Puffball | Diane | hamster | f | 1999-03-30 | NULL |
+----------+-------+---------+------+------------+-------+
You can combine conditions, for example, to locate female
dogs:
mysql> SELECT * FROM pet WHERE species = 'dog' AND sex = 'f';
+-------+--------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+-------+--------+---------+------+------------+-------+
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
+-------+--------+---------+------+------------+-------+
The preceding query uses the AND
logical operator. There is also an
OR operator:
mysql> SELECT * FROM pet WHERE species = 'snake' OR species = 'bird';
+----------+-------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+----------+-------+---------+------+------------+-------+
| Chirpy | Gwen | bird | f | 1998-09-11 | NULL |
| Whistler | Gwen | bird | NULL | 1997-12-09 | NULL |
| Slim | Benny | snake | m | 1996-04-29 | NULL |
+----------+-------+---------+------+------------+-------+
AND and
OR may be intermixed, although
AND has higher precedence than
OR. If you use both operators, it
is a good idea to use parentheses to indicate explicitly how
conditions should be grouped:
mysql> SELECT * FROM pet WHERE (species = 'cat' AND sex = 'm')
-> OR (species = 'dog' AND sex = 'f');
+-------+--------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen | cat | m | 1994-03-17 | NULL |
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
+-------+--------+---------+------+------------+-------+
3.3.4.3. Selecting Particular Columns
If you do not want to see entire rows from your table, just
name the columns in which you are interested, separated by
commas. For example, if you want to know when your animals
were born, select the name and
birth columns:
mysql> SELECT name, birth FROM pet;
+----------+------------+
| name | birth |
+----------+------------+
| Fluffy | 1993-02-04 |
| Claws | 1994-03-17 |
| Buffy | 1989-05-13 |
| Fang | 1990-08-27 |
| Bowser | 1989-08-31 |
| Chirpy | 1998-09-11 |
| Whistler | 1997-12-09 |
| Slim | 1996-04-29 |
| Puffball | 1999-03-30 |
+----------+------------+
To find out who owns pets, use this query:
mysql> SELECT owner FROM pet;
+--------+
| owner |
+--------+
| Harold |
| Gwen |
| Harold |
| Benny |
| Diane |
| Gwen |
| Gwen |
| Benny |
| Diane |
+--------+
Notice that the query simply retrieves the
owner column from each record, and some of
them appear more than once. To minimize the output, retrieve
each unique output record just once by adding the keyword
DISTINCT:
mysql> SELECT DISTINCT owner FROM pet;
+--------+
| owner |
+--------+
| Benny |
| Diane |
| Gwen |
| Harold |
+--------+
You can use a WHERE clause to combine row
selection with column selection. For example, to get birth
dates for dogs and cats only, use this query:
mysql> SELECT name, species, birth FROM pet
-> WHERE species = 'dog' OR species = 'cat';
+--------+---------+------------+
| name | species | birth |
+--------+---------+------------+
| Fluffy | cat | 1993-02-04 |
| Claws | cat | 1994-03-17 |
| Buffy | dog | 1989-05-13 |
| Fang | dog | 1990-08-27 |
| Bowser | dog | 1989-08-31 |
+--------+---------+------------+
You may have noticed in the preceding examples that the result
rows are displayed in no particular order. It is often easier
to examine query output when the rows are sorted in some
meaningful way. To sort a result, use an ORDER
BY clause.
Here are animal birthdays, sorted by date:
mysql> SELECT name, birth FROM pet ORDER BY birth;
+----------+------------+
| name | birth |
+----------+------------+
| Buffy | 1989-05-13 |
| Bowser | 1989-08-31 |
| Fang | 1990-08-27 |
| Fluffy | 1993-02-04 |
| Claws | 1994-03-17 |
| Slim | 1996-04-29 |
| Whistler | 1997-12-09 |
| Chirpy | 1998-09-11 |
| Puffball | 1999-03-30 |
+----------+------------+
On character type columns, sorting — like all other
comparison operations — is normally performed in a
case-insensitive fashion. This means that the order is
undefined for columns that are identical except for their
case. You can force a case-sensitive sort for a column by
using BINARY like so:
ORDER BY BINARY
col_name.
The default sort order is ascending, with smallest values
first. To sort in reverse (descending) order, add the
DESC keyword to the name of the column you
are sorting by:
mysql> SELECT name, birth FROM pet ORDER BY birth DESC;
+----------+------------+
| name | birth |
+----------+------------+
| Puffball | 1999-03-30 |
| Chirpy | 1998-09-11 |
| Whistler | 1997-12-09 |
| Slim | 1996-04-29 |
| Claws | 1994-03-17 |
| Fluffy | 1993-02-04 |
| Fang | 1990-08-27 |
| Bowser | 1989-08-31 |
| Buffy | 1989-05-13 |
+----------+------------+
You can sort on multiple columns, and you can sort different
columns in different directions. For example, to sort by type
of animal in ascending order, then by birth date within animal
type in descending order (youngest animals first), use the
following query:
mysql> SELECT name, species, birth FROM pet
-> ORDER BY species, birth DESC;
+----------+---------+------------+
| name | species | birth |
+----------+---------+------------+
| Chirpy | bird | 1998-09-11 |
| Whistler | bird | 1997-12-09 |
| Claws | cat | 1994-03-17 |
| Fluffy | cat | 1993-02-04 |
| Fang | dog | 1990-08-27 |
| Bowser | dog | 1989-08-31 |
| Buffy | dog | 1989-05-13 |
| Puffball | hamster | 1999-03-30 |
| Slim | snake | 1996-04-29 |
+----------+---------+------------+
The DESC keyword applies only to the column
name immediately preceding it (birth); it
does not affect the species column sort
order.
3.3.4.5. Date Calculations
MySQL provides several functions that you can use to perform
calculations on dates, for example, to calculate ages or
extract parts of dates.
To determine how many years old each of your pets is, compute
the difference in the year part of the current date and the
birth date, then subtract one if the current date occurs
earlier in the calendar year than the birth date. The
following query shows, for each pet, the birth date, the
current date, and the age in years.
mysql> SELECT name, birth, CURDATE(),
-> (YEAR(CURDATE())-YEAR(birth))
-> - (RIGHT(CURDATE(),5)<RIGHT(birth,5))
-> AS age
-> FROM pet;
+----------+------------+------------+------+
| name | birth | CURDATE() | age |
+----------+------------+------------+------+
| Fluffy | 1993-02-04 | 2003-08-19 | 10 |
| Claws | 1994-03-17 | 2003-08-19 | 9 |
| Buffy | 1989-05-13 | 2003-08-19 | 14 |
| Fang | 1990-08-27 | 2003-08-19 | 12 |
| Bowser | 1989-08-31 | 2003-08-19 | 13 |
| Chirpy | 1998-09-11 | 2003-08-19 | 4 |
| Whistler | 1997-12-09 | 2003-08-19 | 5 |
| Slim | 1996-04-29 | 2003-08-19 | 7 |
| Puffball | 1999-03-30 | 2003-08-19 | 4 |
+----------+------------+------------+------+
Here, YEAR() pulls out the year
part of a date and RIGHT()
pulls off the rightmost five characters that represent the
MM-DD (calendar year) part of the date. The
part of the expression that compares the
MM-DD values evaluates to 1 or 0, which
adjusts the year difference down a year if
CURDATE() occurs earlier in the
year than birth. The full expression is
somewhat ungainly, so an alias
(age) is used to make the output column
label more meaningful.
The query works, but the result could be scanned more easily
if the rows were presented in some order. This can be done by
adding an ORDER BY name clause to sort the
output by name:
mysql> SELECT name, birth, CURDATE(),
-> (YEAR(CURDATE())-YEAR(birth))
-> - (RIGHT(CURDATE(),5)<RIGHT(birth,5))
-> AS age
-> FROM pet ORDER BY name;
+----------+------------+------------+------+
| name | birth | CURDATE() | age |
+----------+------------+------------+------+
| Bowser | 1989-08-31 | 2003-08-19 | 13 |
| Buffy | 1989-05-13 | 2003-08-19 | 14 |
| Chirpy | 1998-09-11 | 2003-08-19 | 4 |
| Claws | 1994-03-17 | 2003-08-19 | 9 |
| Fang | 1990-08-27 | 2003-08-19 | 12 |
| Fluffy | 1993-02-04 | 2003-08-19 | 10 |
| Puffball | 1999-03-30 | 2003-08-19 | 4 |
| Slim | 1996-04-29 | 2003-08-19 | 7 |
| Whistler | 1997-12-09 | 2003-08-19 | 5 |
+----------+------------+------------+------+
To sort the output by age rather than
name, just use a different ORDER
BY clause:
mysql> SELECT name, birth, CURDATE(),
-> (YEAR(CURDATE())-YEAR(birth))
-> - (RIGHT(CURDATE(),5)<RIGHT(birth,5))
-> AS age
-> FROM pet ORDER BY age;
+----------+------------+------------+------+
| name | birth | CURDATE() | age |
+----------+------------+------------+------+
| Chirpy | 1998-09-11 | 2003-08-19 | 4 |
| Puffball | 1999-03-30 | 2003-08-19 | 4 |
| Whistler | 1997-12-09 | 2003-08-19 | 5 |
| Slim | 1996-04-29 | 2003-08-19 | 7 |
| Claws | 1994-03-17 | 2003-08-19 | 9 |
| Fluffy | 1993-02-04 | 2003-08-19 | 10 |
| Fang | 1990-08-27 | 2003-08-19 | 12 |
| Bowser | 1989-08-31 | 2003-08-19 | 13 |
| Buffy | 1989-05-13 | 2003-08-19 | 14 |
+----------+------------+------------+------+
A similar query can be used to determine age at death for
animals that have died. You determine which animals these are
by checking whether the death value is
NULL. Then, for those with
non-NULL values, compute the difference
between the death and
birth values:
mysql> SELECT name, birth, death,
-> (YEAR(death)-YEAR(birth)) - (RIGHT(death,5)<RIGHT(birth,5))
-> AS age
-> FROM pet WHERE death IS NOT NULL ORDER BY age;
+--------+------------+------------+------+
| name | birth | death | age |
+--------+------------+------------+------+
| Bowser | 1989-08-31 | 1995-07-29 | 5 |
+--------+------------+------------+------+
The query uses death IS NOT NULL rather
than death <> NULL because
NULL is a special value that cannot be
compared using the usual comparison operators. This is
discussed later. See Section 3.3.4.6, “Working with NULL Values”.
What if you want to know which animals have birthdays next
month? For this type of calculation, year and day are
irrelevant; you simply want to extract the month part of the
birth column. MySQL provides several
functions for extracting parts of dates, such as
YEAR(),
MONTH(), and
DAYOFMONTH().
MONTH() is the appropriate
function here. To see how it works, run a simple query that
displays the value of both birth and
MONTH(birth):
mysql> SELECT name, birth, MONTH(birth) FROM pet;
+----------+------------+--------------+
| name | birth | MONTH(birth) |
+----------+------------+--------------+
| Fluffy | 1993-02-04 | 2 |
| Claws | 1994-03-17 | 3 |
| Buffy | 1989-05-13 | 5 |
| Fang | 1990-08-27 | 8 |
| Bowser | 1989-08-31 | 8 |
| Chirpy | 1998-09-11 | 9 |
| Whistler | 1997-12-09 | 12 |
| Slim | 1996-04-29 | 4 |
| Puffball | 1999-03-30 | 3 |
+----------+------------+--------------+
Finding animals with birthdays in the upcoming month is also
simple. Suppose that the current month is April. Then the
month value is 4 and you can look for
animals born in May (month 5) like this:
mysql> SELECT name, birth FROM pet WHERE MONTH(birth) = 5;
+-------+------------+
| name | birth |
+-------+------------+
| Buffy | 1989-05-13 |
+-------+------------+
There is a small complication if the current month is
December. You cannot merely add one to the month number
(12) and look for animals born in month
13, because there is no such month.
Instead, you look for animals born in January (month
1).
You can write the query so that it works no matter what the
current month is, so that you do not have to use the number
for a particular month.
DATE_ADD() allows you to add a
time interval to a given date. If you add a month to the value
of CURDATE(), then extract the
month part with MONTH(), the
result produces the month in which to look for birthdays:
mysql> SELECT name, birth FROM pet
-> WHERE MONTH(birth) = MONTH(DATE_ADD(CURDATE(),INTERVAL 1 MONTH));
A different way to accomplish the same task is to add
1 to get the next month after the current
one after using the modulo function (MOD)
to wrap the month value to 0 if it is
currently 12:
mysql> SELECT name, birth FROM pet
-> WHERE MONTH(birth) = MOD(MONTH(CURDATE()), 12) + 1;
MONTH() returns a number
between 1 and 12. And
MOD(something,12) returns a
number between 0 and 11.
So the addition has to be after the
MOD(), otherwise we would go
from November (11) to January
(1).
3.3.4.6. Working with NULL Values
The NULL value can be surprising until you
get used to it. Conceptually, NULL means
“a missing unknown value” and it is treated
somewhat differently from other values. To test for
NULL, you cannot use the arithmetic
comparison operators such as =,
<, or <>. To
demonstrate this for yourself, try the following query:
mysql> SELECT 1 = NULL, 1 <> NULL, 1 < NULL, 1 > NULL;
+----------+-----------+----------+----------+
| 1 = NULL | 1 <> NULL | 1 < NULL | 1 > NULL |
+----------+-----------+----------+----------+
| NULL | NULL | NULL | NULL |
+----------+-----------+----------+----------+
Clearly you get no meaningful results from these comparisons.
Use the IS NULL and
IS NOT NULL operators instead:
mysql> SELECT 1 IS NULL, 1 IS NOT NULL;
+-----------+---------------+
| 1 IS NULL | 1 IS NOT NULL |
+-----------+---------------+
| 0 | 1 |
+-----------+---------------+
In MySQL, 0 or NULL
means false and anything else means true. The default truth
value from a boolean operation is 1.
This special treatment of NULL is why, in
the previous section, it was necessary to determine which
animals are no longer alive using death IS NOT
NULL instead of death <>
NULL.
Two NULL values are regarded as equal in a
GROUP BY.
When doing an ORDER BY,
NULL values are presented first if you do
ORDER BY ... ASC and last if you do
ORDER BY ... DESC.
A common error when working with NULL is to
assume that it is not possible to insert a zero or an empty
string into a column defined as NOT NULL,
but this is not the case. These are in fact values, whereas
NULL means “not having a
value.” You can test this easily enough by using
IS [NOT]
NULL as shown:
mysql> SELECT 0 IS NULL, 0 IS NOT NULL, '' IS NULL, '' IS NOT NULL;
+-----------+---------------+------------+----------------+
| 0 IS NULL | 0 IS NOT NULL | '' IS NULL | '' IS NOT NULL |
+-----------+---------------+------------+----------------+
| 0 | 1 | 0 | 1 |
+-----------+---------------+------------+----------------+
Thus it is entirely possible to insert a zero or empty string
into a NOT NULL column, as these are in
fact NOT NULL. See
Section B.5.5.3, “Problems with NULL Values”.
3.3.4.7. Pattern Matching
MySQL provides standard SQL pattern matching as well as a form
of pattern matching based on extended regular expressions
similar to those used by Unix utilities such as
vi, grep, and
sed.
SQL pattern matching allows you to use
“_” to match any single
character and “%” to match an
arbitrary number of characters (including zero characters). In
MySQL, SQL patterns are case-insensitive by default. Some
examples are shown here. You do not use =
or <> when you use SQL patterns; use
the LIKE or
NOT LIKE comparison operators
instead.
To find names beginning with
“b”:
mysql> SELECT * FROM pet WHERE name LIKE 'b%';
+--------+--------+---------+------+------------+------------+
| name | owner | species | sex | birth | death |
+--------+--------+---------+------+------------+------------+
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
| Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 |
+--------+--------+---------+------+------------+------------+
To find names ending with
“fy”:
mysql> SELECT * FROM pet WHERE name LIKE '%fy';
+--------+--------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+--------+--------+---------+------+------------+-------+
| Fluffy | Harold | cat | f | 1993-02-04 | NULL |
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
+--------+--------+---------+------+------------+-------+
To find names containing a
“w”:
mysql> SELECT * FROM pet WHERE name LIKE '%w%';
+----------+-------+---------+------+------------+------------+
| name | owner | species | sex | birth | death |
+----------+-------+---------+------+------------+------------+
| Claws | Gwen | cat | m | 1994-03-17 | NULL |
| Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 |
| Whistler | Gwen | bird | NULL | 1997-12-09 | NULL |
+----------+-------+---------+------+------------+------------+
To find names containing exactly five characters, use five
instances of the “_” pattern
character:
mysql> SELECT * FROM pet WHERE name LIKE '_____';
+-------+--------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen | cat | m | 1994-03-17 | NULL |
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
+-------+--------+---------+------+------------+-------+
The other type of pattern matching provided by MySQL uses
extended regular expressions. When you test for a match for
this type of pattern, use the
REGEXP and NOT
REGEXP operators (or
RLIKE and
NOT RLIKE,
which are synonyms).
The following list describes some characteristics of extended
regular expressions:
“.” matches any single
character.
A character class “[...]”
matches any character within the brackets. For example,
“[abc]” matches
“a”,
“b”, or
“c”. To name a range of
characters, use a dash.
“[a-z]” matches any
letter, whereas “[0-9]”
matches any digit.
“*” matches zero or more
instances of the thing preceding it. For example,
“x*” matches any number of
“x” characters,
“[0-9]*” matches any
number of digits, and “.*”
matches any number of anything.
A REGEXP pattern match
succeeds if the pattern matches anywhere in the value
being tested. (This differs from a
LIKE pattern match, which
succeeds only if the pattern matches the entire value.)
To anchor a pattern so that it must match the beginning or
end of the value being tested, use
“^” at the beginning or
“$” at the end of the
pattern.
To demonstrate how extended regular expressions work, the
LIKE queries shown previously are
rewritten here to use REGEXP.
To find names beginning with
“b”, use
“^” to match the beginning of
the name:
mysql> SELECT * FROM pet WHERE name REGEXP '^b';
+--------+--------+---------+------+------------+------------+
| name | owner | species | sex | birth | death |
+--------+--------+---------+------+------------+------------+
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
| Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 |
+--------+--------+---------+------+------------+------------+
If you really want to force a
REGEXP comparison to be case
sensitive, use the BINARY keyword
to make one of the strings a binary string. This query matches
only lowercase “b” at the
beginning of a name:
mysql> SELECT * FROM pet WHERE name REGEXP BINARY '^b';
To find names ending with
“fy”, use
“$” to match the end of the
name:
mysql> SELECT * FROM pet WHERE name REGEXP 'fy$';
+--------+--------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+--------+--------+---------+------+------------+-------+
| Fluffy | Harold | cat | f | 1993-02-04 | NULL |
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
+--------+--------+---------+------+------------+-------+
To find names containing a
“w”, use this query:
mysql> SELECT * FROM pet WHERE name REGEXP 'w';
+----------+-------+---------+------+------------+------------+
| name | owner | species | sex | birth | death |
+----------+-------+---------+------+------------+------------+
| Claws | Gwen | cat | m | 1994-03-17 | NULL |
| Bowser | Diane | dog | m | 1989-08-31 | 1995-07-29 |
| Whistler | Gwen | bird | NULL | 1997-12-09 | NULL |
+----------+-------+---------+------+------------+------------+
Because a regular expression pattern matches if it occurs
anywhere in the value, it is not necessary in the previous
query to put a wildcard on either side of the pattern to get
it to match the entire value like it would be if you used an
SQL pattern.
To find names containing exactly five characters, use
“^” and
“$” to match the beginning and
end of the name, and five instances of
“.” in between:
mysql> SELECT * FROM pet WHERE name REGEXP '^.....$';
+-------+--------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen | cat | m | 1994-03-17 | NULL |
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
+-------+--------+---------+------+------------+-------+
You could also write the previous query using the
{n}
(“repeat-n-times”)
operator:
mysql> SELECT * FROM pet WHERE name REGEXP '^.{5}$';
+-------+--------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen | cat | m | 1994-03-17 | NULL |
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
+-------+--------+---------+------+------------+-------+
Section 11.4.2, “Regular Expressions”, provides more information about the
syntax for regular expressions.
Databases are often used to answer the question, “How
often does a certain type of data occur in a table?”
For example, you might want to know how many pets you have, or
how many pets each owner has, or you might want to perform
various kinds of census operations on your animals.
Counting the total number of animals you have is the same
question as “How many rows are in the
pet table?” because there is one
record per pet. COUNT(*) counts
the number of rows, so the query to count your animals looks
like this:
mysql> SELECT COUNT(*) FROM pet;
+----------+
| COUNT(*) |
+----------+
| 9 |
+----------+
Earlier, you retrieved the names of the people who owned pets.
You can use COUNT() if you want
to find out how many pets each owner has:
mysql> SELECT owner, COUNT(*) FROM pet GROUP BY owner;
+--------+----------+
| owner | COUNT(*) |
+--------+----------+
| Benny | 2 |
| Diane | 2 |
| Gwen | 3 |
| Harold | 2 |
+--------+----------+
The preceding query uses GROUP BY to group
all records for each owner. The use of
COUNT() in conjunction with
GROUP BY is useful for characterizing your
data under various groupings. The following examples show
different ways to perform animal census operations.
Number of animals per species:
mysql> SELECT species, COUNT(*) FROM pet GROUP BY species;
+---------+----------+
| species | COUNT(*) |
+---------+----------+
| bird | 2 |
| cat | 2 |
| dog | 3 |
| hamster | 1 |
| snake | 1 |
+---------+----------+
Number of animals per sex:
mysql> SELECT sex, COUNT(*) FROM pet GROUP BY sex;
+------+----------+
| sex | COUNT(*) |
+------+----------+
| NULL | 1 |
| f | 4 |
| m | 4 |
+------+----------+
(In this output, NULL indicates that the
sex is unknown.)
Number of animals per combination of species and sex:
mysql> SELECT species, sex, COUNT(*) FROM pet GROUP BY species, sex;
+---------+------+----------+
| species | sex | COUNT(*) |
+---------+------+----------+
| bird | NULL | 1 |
| bird | f | 1 |
| cat | f | 1 |
| cat | m | 1 |
| dog | f | 1 |
| dog | m | 2 |
| hamster | f | 1 |
| snake | m | 1 |
+---------+------+----------+
You need not retrieve an entire table when you use
COUNT(). For example, the
previous query, when performed just on dogs and cats, looks
like this:
mysql> SELECT species, sex, COUNT(*) FROM pet
-> WHERE species = 'dog' OR species = 'cat'
-> GROUP BY species, sex;
+---------+------+----------+
| species | sex | COUNT(*) |
+---------+------+----------+
| cat | f | 1 |
| cat | m | 1 |
| dog | f | 1 |
| dog | m | 2 |
+---------+------+----------+
Or, if you wanted the number of animals per sex only for
animals whose sex is known:
mysql> SELECT species, sex, COUNT(*) FROM pet
-> WHERE sex IS NOT NULL
-> GROUP BY species, sex;
+---------+------+----------+
| species | sex | COUNT(*) |
+---------+------+----------+
| bird | f | 1 |
| cat | f | 1 |
| cat | m | 1 |
| dog | f | 1 |
| dog | m | 2 |
| hamster | f | 1 |
| snake | m | 1 |
+---------+------+----------+
If you name columns to select in addition to the
COUNT() value, a GROUP
BY clause should be present that names those same
columns. Otherwise, the following occurs:
If the
ONLY_FULL_GROUP_BY SQL
mode is enabled, an error occurs:
mysql> SET sql_mode = 'ONLY_FULL_GROUP_BY';
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT owner, COUNT(*) FROM pet;
ERROR 1140 (42000): Mixing of GROUP columns (MIN(),MAX(),COUNT()...)
with no GROUP columns is illegal if there is no GROUP BY clause
If ONLY_FULL_GROUP_BY is
not enabled, the query is processed by treating all rows
as a single group, but the value selected for each named
column is indeterminate. The server is free to select the
value from any row:
mysql> SET sql_mode = '';
Query OK, 0 rows affected (0.00 sec)
mysql> SELECT owner, COUNT(*) FROM pet;
+--------+----------+
| owner | COUNT(*) |
+--------+----------+
| Harold | 8 |
+--------+----------+
1 row in set (0.00 sec)
See also Section 11.11.3, “GROUP BY and HAVING with Hidden
Columns”.
3.3.4.9. Using More Than one Table
The pet table keeps track of which pets you
have. If you want to record other information about them, such
as events in their lives like visits to the vet or when
litters are born, you need another table. What should this
table look like? It needs to contain the following
information:
The pet name so that you know which animal each event
pertains to.
A date so that you know when the event occurred.
A field to describe the event.
An event type field, if you want to be able to categorize
events.
Given these considerations, the CREATE
TABLE statement for the event
table might look like this:
mysql> CREATE TABLE event (name VARCHAR(20), date DATE,
-> type VARCHAR(15), remark VARCHAR(255));
As with the pet table, it is easiest to
load the initial records by creating a tab-delimited text file
containing the following information.
Load the records like this:
mysql> LOAD DATA LOCAL INFILE 'event.txt' INTO TABLE event;
Based on what you have learned from the queries that you have
run on the pet table, you should be able to
perform retrievals on the records in the
event table; the principles are the same.
But when is the event table by itself
insufficient to answer questions you might ask?
Suppose that you want to find out the ages at which each pet
had its litters. We saw earlier how to calculate ages from two
dates. The litter date of the mother is in the
event table, but to calculate her age on
that date you need her birth date, which is stored in the
pet table. This means the query requires
both tables:
mysql> SELECT pet.name,
-> (YEAR(date)-YEAR(birth)) - (RIGHT(date,5)<RIGHT(birth,5)) AS age,
-> remark
-> FROM pet INNER JOIN event
-> ON pet.name = event.name
-> WHERE event.type = 'litter';
+--------+------+-----------------------------+
| name | age | remark |
+--------+------+-----------------------------+
| Fluffy | 2 | 4 kittens, 3 female, 1 male |
| Buffy | 4 | 5 puppies, 2 female, 3 male |
| Buffy | 5 | 3 puppies, 3 female |
+--------+------+-----------------------------+
There are several things to note about this query:
The FROM clause joins two tables
because the query needs to pull information from both of
them.
When combining (joining) information from multiple tables,
you need to specify how records in one table can be
matched to records in the other. This is easy because they
both have a name column. The query uses
ON clause to match up records in the
two tables based on the name values.
The query uses an INNER JOIN to combine
the tables. An INNER JOIN allows for
rows from either table to appear in the result if and only
if both tables meet the conditions specified in the
ON clause. In this example, the
ON clause specifies that the
name column in the
pet table must match the
name column in the
event table. If a name appears in one
table but not the other, the row will not appear in the
result because the condition in the ON
clause fails.
Because the name column occurs in both
tables, you must be specific about which table you mean
when referring to the column. This is done by prepending
the table name to the column name.
You need not have two different tables to perform a join.
Sometimes it is useful to join a table to itself, if you want
to compare records in a table to other records in that same
table. For example, to find breeding pairs among your pets,
you can join the pet table with itself to
produce candidate pairs of males and females of like species:
mysql> SELECT p1.name, p1.sex, p2.name, p2.sex, p1.species
-> FROM pet AS p1 INNER JOIN pet AS p2
-> ON p1.species = p2.species AND p1.sex = 'f' AND p2.sex = 'm';
+--------+------+--------+------+---------+
| name | sex | name | sex | species |
+--------+------+--------+------+---------+
| Fluffy | f | Claws | m | cat |
| Buffy | f | Fang | m | dog |
| Buffy | f | Bowser | m | dog |
+--------+------+--------+------+---------+
In this query, we specify aliases for the table name to refer
to the columns and keep straight which instance of the table
each column reference is associated with.
3.4. Getting Information About Databases and Tables
What if you forget the name of a database or table, or what the
structure of a given table is (for example, what its columns are
called)? MySQL addresses this problem through several statements
that provide information about the databases and tables it
supports.
You have previously seen SHOW
DATABASES, which lists the databases managed by the
server. To find out which database is currently selected, use the
DATABASE() function:
mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| menagerie |
+------------+
If you have not yet selected any database, the result is
NULL.
To find out what tables the default database contains (for
example, when you are not sure about the name of a table), use
this command:
mysql> SHOW TABLES;
+---------------------+
| Tables_in_menagerie |
+---------------------+
| event |
| pet |
+---------------------+
The name of the column in the output produced by this statement is
always
Tables_in_db_name,
where db_name is the name of the
database. See Section 12.5.5.34, “SHOW TABLES Syntax”, for more information.
If you want to find out about the structure of a table, the
DESCRIBE command is useful; it
displays information about each of a table's columns:
mysql> DESCRIBE pet;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name | varchar(20) | YES | | NULL | |
| owner | varchar(20) | YES | | NULL | |
| species | varchar(20) | YES | | NULL | |
| sex | char(1) | YES | | NULL | |
| birth | date | YES | | NULL | |
| death | date | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+
Field indicates the column name,
Type is the data type for the column,
NULL indicates whether the column can contain
NULL values, Key indicates
whether the column is indexed, and Default
specifies the column's default value. Extra
displays special information about columns: If a column was
created with the AUTO_INCREMENT option, the
value will be auto_increment rather than empty.
DESC is a short form of
DESCRIBE. See
Section 12.3.1, “DESCRIBE Syntax”, for more information.
You can obtain the CREATE TABLE
statement necessary to create an existing table using the
SHOW CREATE TABLE statement. See
Section 12.5.5.9, “SHOW CREATE TABLE Syntax”.
If you have indexes on a table, SHOW INDEX FROM
tbl_name produces information
about them. See Section 12.5.5.18, “SHOW INDEX Syntax”, for more about this
statement.
3.5. Using mysql in Batch Mode
In the previous sections, you used mysql
interactively to enter queries and view the results. You can also
run mysql in batch mode. To do this, put the
commands you want to run in a file, then tell
mysql to read its input from the file:
shell> mysql < batch-file
If you are running mysql under Windows and have
some special characters in the file that cause problems, you can
do this:
C:\> mysql -e "source batch-file"
If you need to specify connection parameters on the command line,
the command might look like this:
shell> mysql -h host -u user -p < batch-file
Enter password: ********
When you use mysql this way, you are creating a
script file, then executing the script.
If you want the script to continue even if some of the statements
in it produce errors, you should use the
--force command-line option.
Why use a script? Here are a few reasons:
If you run a query repeatedly (say, every day or every week),
making it a script allows you to avoid retyping it each time
you execute it.
You can generate new queries from existing ones that are
similar by copying and editing script files.
Batch mode can also be useful while you're developing a query,
particularly for multiple-line commands or multiple-statement
sequences of commands. If you make a mistake, you don't have
to retype everything. Just edit your script to correct the
error, then tell mysql to execute it again.
If you have a query that produces a lot of output, you can run
the output through a pager rather than watching it scroll off
the top of your screen:
shell> mysql < batch-file | more
You can catch the output in a file for further processing:
shell> mysql < batch-file > mysql.out
You can distribute your script to other people so that they
can also run the commands.
Some situations do not allow for interactive use, for example,
when you run a query from a cron job. In
this case, you must use batch mode.
The default output format is different (more concise) when you run
mysql in batch mode than when you use it
interactively. For example, the output of SELECT DISTINCT
species FROM pet looks like this when
mysql is run interactively:
+---------+
| species |
+---------+
| bird |
| cat |
| dog |
| hamster |
| snake |
+---------+
In batch mode, the output looks like this instead:
species
bird
cat
dog
hamster
snake
If you want to get the interactive output format in batch mode,
use mysql -t. To echo to the output the
commands that are executed, use mysql -vvv.
You can also use scripts from the mysql prompt
by using the source command or
\. command:
mysql> source filename;
mysql> \. filename
See Section 4.5.1.4, “Executing SQL Statements from a Text File”, for more information.
3.6. Examples of Common Queries
Here are examples of how to solve some common problems with MySQL.
Some of the examples use the table shop to hold
the price of each article (item number) for certain traders
(dealers). Supposing that each trader has a single fixed price per
article, then (article,
dealer) is a primary key for the records.
Start the command-line tool mysql and select a
database:
shell> mysql your-database-name
(In most MySQL installations, you can use the database named
test).
You can create and populate the example table with these
statements:
CREATE TABLE shop (
article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
dealer CHAR(20) DEFAULT '' NOT NULL,
price DOUBLE(16,2) DEFAULT '0.00' NOT NULL,
PRIMARY KEY(article, dealer));
INSERT INTO shop VALUES
(1,'A',3.45),(1,'B',3.99),(2,'A',10.99),(3,'B',1.45),
(3,'C',1.69),(3,'D',1.25),(4,'D',19.95);
After issuing the statements, the table should have the following
contents:
SELECT * FROM shop;
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
| 0001 | A | 3.45 |
| 0001 | B | 3.99 |
| 0002 | A | 10.99 |
| 0003 | B | 1.45 |
| 0003 | C | 1.69 |
| 0003 | D | 1.25 |
| 0004 | D | 19.95 |
+---------+--------+-------+
3.6.1. The Maximum Value for a Column
“What is the highest item number?”
SELECT MAX(article) AS article FROM shop;
+---------+
| article |
+---------+
| 4 |
+---------+
3.6.2. The Row Holding the Maximum of a Certain Column
Task: Find the number, dealer, and price of the most
expensive article.
This is easily done with a subquery:
SELECT article, dealer, price
FROM shop
WHERE price=(SELECT MAX(price) FROM shop);
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
| 0004 | D | 19.95 |
+---------+--------+-------+
Other solutions are to use a LEFT JOIN or to
sort all rows descending by price and get only the first row
using the MySQL-specific LIMIT clause:
SELECT s1.article, s1.dealer, s1.price
FROM shop s1
LEFT JOIN shop s2 ON s1.price < s2.price
WHERE s2.article IS NULL;
SELECT article, dealer, price
FROM shop
ORDER BY price DESC
LIMIT 1;
Note
If there were several most expensive articles, each with a
price of 19.95, the LIMIT solution would
show only one of them.
3.6.3. Maximum of Column per Group
Task: Find the highest price per article.
SELECT article, MAX(price) AS price
FROM shop
GROUP BY article;
+---------+-------+
| article | price |
+---------+-------+
| 0001 | 3.99 |
| 0002 | 10.99 |
| 0003 | 1.69 |
| 0004 | 19.95 |
+---------+-------+
3.6.4. The Rows Holding the Group-wise Maximum of a Certain Column
Task: For each article, find the dealer or dealers
with the most expensive price.
This problem can be solved with a subquery like this one:
SELECT article, dealer, price
FROM shop s1
WHERE price=(SELECT MAX(s2.price)
FROM shop s2
WHERE s1.article = s2.article);
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
| 0001 | B | 3.99 |
| 0002 | A | 10.99 |
| 0003 | C | 1.69 |
| 0004 | D | 19.95 |
+---------+--------+-------+
The preceding example uses a correlated subquery, which can be
inefficient (see Section 12.2.9.7, “Correlated Subqueries”). Other
possibilities for solving the problem are to use an uncorrelated
subquery in the FROM clause or a
LEFT JOIN:
SELECT s1.article, dealer, s1.price
FROM shop s1
JOIN (
SELECT article, MAX(price) AS price
FROM shop
GROUP BY article) AS s2
ON s1.article = s2.article AND s1.price = s2.price;
SELECT s1.article, s1.dealer, s1.price
FROM shop s1
LEFT JOIN shop s2 ON s1.article = s2.article AND s1.price < s2.price
WHERE s2.article IS NULL;
The LEFT JOIN works on the basis that when
s1.price is at its maximum value, there is no
s2.price with a greater value and the
s2 rows values will be
NULL. See Section 12.2.8.1, “JOIN Syntax”.
3.6.5. Using User-Defined Variables
You can employ MySQL user variables to remember results without
having to store them in temporary variables in the client. (See
Section 8.4, “User-Defined Variables”.)
For example, to find the articles with the highest and lowest
price you can do this:
mysql> SELECT @min_price:=MIN(price),@max_price:=MAX(price) FROM shop;
mysql> SELECT * FROM shop WHERE price=@min_price OR price=@max_price;
+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
| 0003 | D | 1.25 |
| 0004 | D | 19.95 |
+---------+--------+-------+
Note
It is also possible to store the name of a database object
such as a table or a column in a user variable and then to use
this variable in an SQL statement; however, this requires the
use of a prepared statement. See
Section 12.7, “SQL Syntax for Prepared Statements”, for more
information.
3.6.6. Using Foreign Keys
In MySQL, InnoDB tables support checking of
foreign key constraints. See Section 13.2, “The InnoDB Storage Engine”, and
Section 1.8.5.4, “Foreign Keys”.
A foreign key constraint is not required merely to join two
tables. For storage engines other than
InnoDB, it is possible when defining a column
to use a REFERENCES
tbl_name(col_name)
clause, which has no actual effect, and serves only as
a memo or comment to you that the column which you are currently
defining is intended to refer to a column in another
table. It is extremely important to realize when
using this syntax that:
MySQL does not perform any sort of CHECK
to make sure that col_name
actually exists in tbl_name (or
even that tbl_name itself
exists).
MySQL does not perform any sort of action on
tbl_name such as deleting rows in
response to actions taken on rows in the table which you are
defining; in other words, this syntax induces no ON
DELETE or ON UPDATE behavior
whatsoever. (Although you can write an ON
DELETE or ON UPDATE clause as
part of the REFERENCES clause, it is also
ignored.)
This syntax creates a column; it does
not create any sort of
index or key.
You can use a column so created as a join column, as shown here:
CREATE TABLE person (
id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
name CHAR(60) NOT NULL,
PRIMARY KEY (id)
);
CREATE TABLE shirt (
id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
style ENUM('t-shirt', 'polo', 'dress') NOT NULL,
color ENUM('red', 'blue', 'orange', 'white', 'black') NOT NULL,
owner SMALLINT UNSIGNED NOT NULL REFERENCES person(id),
PRIMARY KEY (id)
);
INSERT INTO person VALUES (NULL, 'Antonio Paz');
SELECT @last := LAST_INSERT_ID();
INSERT INTO shirt VALUES
(NULL, 'polo', 'blue', @last),
(NULL, 'dress', 'white', @last),
(NULL, 't-shirt', 'blue', @last);
INSERT INTO person VALUES (NULL, 'Lilliana Angelovska');
SELECT @last := LAST_INSERT_ID();
INSERT INTO shirt VALUES
(NULL, 'dress', 'orange', @last),
(NULL, 'polo', 'red', @last),
(NULL, 'dress', 'blue', @last),
(NULL, 't-shirt', 'white', @last);
SELECT * FROM person;
+----+---------------------+
| id | name |
+----+---------------------+
| 1 | Antonio Paz |
| 2 | Lilliana Angelovska |
+----+---------------------+
SELECT * FROM shirt;
+----+---------+--------+-------+
| id | style | color | owner |
+----+---------+--------+-------+
| 1 | polo | blue | 1 |
| 2 | dress | white | 1 |
| 3 | t-shirt | blue | 1 |
| 4 | dress | orange | 2 |
| 5 | polo | red | 2 |
| 6 | dress | blue | 2 |
| 7 | t-shirt | white | 2 |
+----+---------+--------+-------+
SELECT s.* FROM person p INNER JOIN shirt s
ON s.owner = p.id
WHERE p.name LIKE 'Lilliana%'
AND s.color <> 'white';
+----+-------+--------+-------+
| id | style | color | owner |
+----+-------+--------+-------+
| 4 | dress | orange | 2 |
| 5 | polo | red | 2 |
| 6 | dress | blue | 2 |
+----+-------+--------+-------+
When used in this fashion, the REFERENCES
clause is not displayed in the output of
SHOW CREATE TABLE or
DESCRIBE:
SHOW CREATE TABLE shirt\G
*************************** 1. row ***************************
Table: shirt
Create Table: CREATE TABLE `shirt` (
`id` smallint(5) unsigned NOT NULL auto_increment,
`style` enum('t-shirt','polo','dress') NOT NULL,
`color` enum('red','blue','orange','white','black') NOT NULL,
`owner` smallint(5) unsigned NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1
The use of REFERENCES in this way as a
comment or “reminder” in a column definition works
with both MyISAM and
BerkeleyDB tables.
3.6.7. Searching on Two Keys
An OR using a single key is well
optimized, as is the handling of
AND.
The one tricky case is that of searching on two different keys
combined with OR:
SELECT field1_index, field2_index FROM test_table
WHERE field1_index = '1' OR field2_index = '1'
This case is optimized from MySQL 5.0.0. See
Section 7.2.6, “Index Merge Optimization”.
You can also solve the problem efficiently by using a
UNION that combines the output of
two separate SELECT statements.
See Section 12.2.8.3, “UNION Syntax”.
Each SELECT searches only one key
and can be optimized:
SELECT field1_index, field2_index
FROM test_table WHERE field1_index = '1'
UNION
SELECT field1_index, field2_index
FROM test_table WHERE field2_index = '1';
3.6.8. Calculating Visits Per Day
The following example shows how you can use the bit group
functions to calculate the number of days per month a user has
visited a Web page.
CREATE TABLE t1 (year YEAR(4), month INT(2) UNSIGNED ZEROFILL,
day INT(2) UNSIGNED ZEROFILL);
INSERT INTO t1 VALUES(2000,1,1),(2000,1,20),(2000,1,30),(2000,2,2),
(2000,2,23),(2000,2,23);
The example table contains year-month-day values representing
visits by users to the page. To determine how many different
days in each month these visits occur, use this query:
SELECT year,month,BIT_COUNT(BIT_OR(1<<day)) AS days FROM t1
GROUP BY year,month;
Which returns:
+------+-------+------+
| year | month | days |
+------+-------+------+
| 2000 | 01 | 3 |
| 2000 | 02 | 2 |
+------+-------+------+
The query calculates how many different days appear in the table
for each year/month combination, with automatic removal of
duplicate entries.
3.6.9. Using AUTO_INCREMENT
The AUTO_INCREMENT attribute can be used to
generate a unique identity for new rows:
CREATE TABLE animals (
id MEDIUMINT NOT NULL AUTO_INCREMENT,
name CHAR(30) NOT NULL,
PRIMARY KEY (id)
);
INSERT INTO animals (name) VALUES
('dog'),('cat'),('penguin'),
('lax'),('whale'),('ostrich');
SELECT * FROM animals;
Which returns:
+----+---------+
| id | name |
+----+---------+
| 1 | dog |
| 2 | cat |
| 3 | penguin |
| 4 | lax |
| 5 | whale |
| 6 | ostrich |
+----+---------+
You can retrieve the most recent
AUTO_INCREMENT value with the
LAST_INSERT_ID() SQL function or
the mysql_insert_id() C API
function. These functions are connection-specific, so their
return values are not affected by another connection which is
also performing inserts.
Note
For a multiple-row insert,
LAST_INSERT_ID() and
mysql_insert_id() actually
return the AUTO_INCREMENT key from the
first of the inserted rows. This allows
multiple-row inserts to be reproduced correctly on other
servers in a replication setup.
For MyISAM and BDB tables
you can specify AUTO_INCREMENT on a secondary
column in a multiple-column index. In this case, the generated
value for the AUTO_INCREMENT column is
calculated as
MAX(auto_increment_column)
+ 1 WHERE
prefix=given-prefix. This
is useful when you want to put data into ordered groups.
CREATE TABLE animals (
grp ENUM('fish','mammal','bird') NOT NULL,
id MEDIUMINT NOT NULL AUTO_INCREMENT,
name CHAR(30) NOT NULL,
PRIMARY KEY (grp,id)
) ENGINE=MyISAM;
INSERT INTO animals (grp,name) VALUES
('mammal','dog'),('mammal','cat'),
('bird','penguin'),('fish','lax'),('mammal','whale'),
('bird','ostrich');
SELECT * FROM animals ORDER BY grp,id;
Which returns:
+--------+----+---------+
| grp | id | name |
+--------+----+---------+
| fish | 1 | lax |
| mammal | 1 | dog |
| mammal | 2 | cat |
| mammal | 3 | whale |
| bird | 1 | penguin |
| bird | 2 | ostrich |
+--------+----+---------+
In this case (when the AUTO_INCREMENT column
is part of a multiple-column index),
AUTO_INCREMENT values are reused if you
delete the row with the biggest
AUTO_INCREMENT value in any group. This
happens even for MyISAM tables, for which
AUTO_INCREMENT values normally are not
reused.
If the AUTO_INCREMENT column is part of
multiple indexes, MySQL will generate sequence values using the
index that begins with the AUTO_INCREMENT
column, if there is one. For example, if the
animals table contained indexes
PRIMARY KEY (grp, id) and INDEX
(id), MySQL would ignore the PRIMARY
KEY for generating sequence values. As a result, the
table would contain a single sequence, not a sequence per
grp value.
To start with an AUTO_INCREMENT value other
than 1, you can set that value with CREATE
TABLE or ALTER TABLE,
like this:
mysql> ALTER TABLE tbl AUTO_INCREMENT = 100;
More information about AUTO_INCREMENT is
available here:
3.7. Queries from the Twin Project
At the places the early MySQL was developed (Analytikerna and
Lentus), the founders did systems and field work for a big
research project. This project was a collaboration between the
Institute of Environmental Medicine at Karolinska Institutet
Stockholm and the Section on Clinical Research in Aging and
Psychology at the University of Southern California.
The project involved lots of data collection from all twins in
Sweden older than 65 Years (see
http://ki.se/ki/jsp/polopoly.jsp?d=9610&l=en).
Large parts of the project were administered with a Web interface
written using Perl and MySQL.
3.7.1. Find All Nondistributed Twins
The following query was used to determine what twins should be
studied further after a initial screening. This was written
around 1997 using MySQL 3.19.
SELECT
CONCAT(p1.id, p1.tvab) + 0 AS tvid,
CONCAT(p1.christian_name, ' ', p1.surname) AS Name,
p1.postal_code AS Code,
p1.city AS City,
pg.abrev AS Area,
IF(td.participation = 'Aborted', 'A', ' ') AS A,
p1.dead AS dead1,
l.event AS event1,
td.suspect AS tsuspect1,
id.suspect AS isuspect1,
td.severe AS tsevere1,
id.severe AS isevere1,
p2.dead AS dead2,
l2.event AS event2,
h2.nurse AS nurse2,
h2.doctor AS doctor2,
td2.suspect AS tsuspect2,
id2.suspect AS isuspect2,
td2.severe AS tsevere2,
id2.severe AS isevere2,
l.finish_date
FROM
twin_project AS tp
/* For Twin 1 */
LEFT JOIN twin_data AS td ON tp.id = td.id
AND tp.tvab = td.tvab
LEFT JOIN informant_data AS id ON tp.id = id.id
AND tp.tvab = id.tvab
LEFT JOIN harmony AS h ON tp.id = h.id
AND tp.tvab = h.tvab
LEFT JOIN lentus AS l ON tp.id = l.id
AND tp.tvab = l.tvab
/* For Twin 2 */
LEFT JOIN twin_data AS td2 ON p2.id = td2.id
AND p2.tvab = td2.tvab
LEFT JOIN informant_data AS id2 ON p2.id = id2.id
AND p2.tvab = id2.tvab
LEFT JOIN harmony AS h2 ON p2.id = h2.id
AND p2.tvab = h2.tvab
LEFT JOIN lentus AS l2 ON p2.id = l2.id
AND p2.tvab = l2.tvab,
person_data AS p1,
person_data AS p2,
postal_groups AS pg
WHERE
/* p1 gets main twin and p2 gets his/her twin. */
/* ptvab is a field inverted from tvab */
p1.id = tp.id AND p1.tvab = tp.tvab AND
p2.id = p1.id AND p2.ptvab = p1.tvab AND
/* Just the screening survey */
tp.survey_no = 5 AND
/* Skip if partner died before 65 but allow emigration (dead=9) */
(p2.dead = 0 OR p2.dead = 9 OR
(p2.dead = 1 AND
(p2.death_date = 0 OR
(((TO_DAYS(p2.death_date) - TO_DAYS(p2.birthday)) / 365)
>= 65))))
AND
(
/* Twin is suspect */
(td.future_contact = 'Yes' AND td.suspect = 2) OR
/* Twin is suspect - Informant is Blessed */
(td.future_contact = 'Yes' AND td.suspect = 1
AND id.suspect = 1) OR
/* No twin - Informant is Blessed */
(ISNULL(td.suspect) AND id.suspect = 1
AND id.future_contact = 'Yes') OR
/* Twin broken off - Informant is Blessed */
(td.participation = 'Aborted'
AND id.suspect = 1 AND id.future_contact = 'Yes') OR
/* Twin broken off - No inform - Have partner */
(td.participation = 'Aborted' AND ISNULL(id.suspect)
AND p2.dead = 0))
AND
l.event = 'Finished'
/* Get at area code */
AND SUBSTRING(p1.postal_code, 1, 2) = pg.code
/* Not already distributed */
AND (h.nurse IS NULL OR h.nurse=00 OR h.doctor=00)
/* Has not refused or been aborted */
AND NOT (h.status = 'Refused' OR h.status = 'Aborted'
OR h.status = 'Died' OR h.status = 'Other')
ORDER BY
tvid;
Some explanations:
CONCAT(p1.id, p1.tvab) + 0 AS
tvid
We want to sort on the concatenated id
and tvab in numerical order. Adding
0 to the result causes MySQL to treat the
result as a number.
Column id
This identifies a pair of twins. It is an index in all
tables.
Column tvab
This identifies a twin in a pair. It has a value of
1 or 2.
Column ptvab
This is an inverse of tvab. When
tvab is 1 this is
2, and vice versa. It exists to save
typing and to make it easier for MySQL to optimize the
query.
This query demonstrates, among other things, how to do lookups
on a table from the same table with a join
(p1 and p2). In the
example, this is used to check whether a twin's partner died
before the age of 65. If so, the row is not returned.
All of the above exist in all tables with twin-related
information. We have an index on both id,
tvab (all tables), and id, ptvab
(person_data) to make queries faster.
When we did this work, our production machine was a 200MHz
UltraSPARC, and on that old hardware this query returned about
150–200 rows in less than one second. The main table had
70k Rows.
3.7.2. Show a Table of Twin Pair Status
Each twin has a status code called event. The
query shown here is used to select all twin pairs combined by
event. This indicates in how many pairs both twins are finished,
in how many pairs one twin is finished and the other refused,
and so on.
SELECT
t1.event,
t2.event,
COUNT(*)
FROM
lentus AS t1,
lentus AS t2,
twin_project AS tp
WHERE
/* We are looking at one pair at a time */
t1.id = tp.id
AND t1.tvab=tp.tvab
AND t1.id = t2.id
/* Just the screening survey */
AND tp.survey_no = 5
/* This makes each pair only appear once */
AND t1.tvab='1' AND t2.tvab='2'
GROUP BY
t1.event, t2.event;
3.8. Using MySQL with Apache
There are programs that let you authenticate your users from a
MySQL database and also let you write your log files into a MySQL
table.
You can change the Apache logging format to be easily readable by
MySQL by putting the following into the Apache configuration file:
LogFormat \
"\"%h\",%{%Y%m%d%H%M%S}t,%>s,\"%b\",\"%{Content-Type}o\", \
\"%U\",\"%{Referer}i\",\"%{User-Agent}i\""
To load a log file in that format into MySQL, you can use a
statement something like this:
LOAD DATA INFILE '/local/access_log' INTO TABLE tbl_name
FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"' ESCAPED BY '\\'
The named table should be created to have columns that correspond
to those that the LogFormat line writes to the
log file.
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