Table of Contents
Once you know how to enter SQL statements, 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:
Create a database
Create a table
Load data into the table
Retrieve data from the table in various ways
Use multiple tables
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 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 statement 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.
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.)
If you get an error such as ERROR 1044 (42000): Access denied for user 'micah'@'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 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 statement:
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 -hEnter password:host-uuser-p menagerie********
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.
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.
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 Data Types.
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.)
| name | owner | species | sex | birth | death |
|---|---|---|---|---|---|
| Fluffy | Harold | cat | f | 1993-02-04 | |
| Claws | Gwen | cat | m | 1994-03-17 | |
| Buffy | Harold | dog | f | 1989-05-13 | |
| Fang | Benny | dog | m | 1990-08-27 | |
| Bowser | Diane | dog | m | 1979-08-31 | 1995-07-29 |
| Chirpy | Gwen | bird | f | 1998-09-11 | |
| Whistler | Gwen | bird | 1997-12-09 | ||
| Slim | Benny | snake | m | 1996-04-29 |
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 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.
The SELECT statement is used to
pull information from a table. The general form of the statement
is:
SELECTwhat_to_selectFROMwhich_tableWHEREconditions_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.
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 '/path/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.
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 | +-------+--------+---------+------+------------+-------+
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.
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, use the
TIMESTAMPDIFF() function. Its
arguments are the unit in which you want the result expressed,
and the two date for which to take the difference. The
following query shows, for each pet, the birth date, the
current date, and the age in years. An
alias (age) is used to
make the final output column label more meaningful.
mysql>SELECT name, birth, CURDATE(),->TIMESTAMPDIFF(YEAR,birth,CURDATE()) 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 | +----------+------------+------------+------+
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(),->TIMESTAMPDIFF(YEAR,birth,CURDATE()) 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(),->TIMESTAMPDIFF(YEAR,birth,CURDATE()) 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,->TIMESTAMPDIFF(YEAR,birth,death) 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 4.4.6, “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() enables 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).
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, use the
IS NULL and IS
NOT NULL operators, as shown here:
mysql> SELECT 1 IS NULL, 1 IS NOT NULL;
+-----------+---------------+
| 1 IS NULL | 1 IS NOT NULL |
+-----------+---------------+
| 0 | 1 |
+-----------+---------------+
You cannot use arithmetic comparison operators such as
=,
<, or
<> to
test for NULL. 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 |
+----------+-----------+----------+----------+
Because the result of any arithmetic comparison with
NULL is also NULL, you
cannot obtain any meaningful results from such comparisons.
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
Problems with NULL Values.
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 enables 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 '^.....$';
+-------+--------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen | cat | m | 1994-03-17 | NULL |
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
+-------+--------+---------+------+------------+-------+
You could also write the previous query using the
{
(“repeat-n}n-times”)
operator:
mysql> SELECT * FROM pet WHERE name REGEXP '^.{5}$';
+-------+--------+---------+------+------------+-------+
| name | owner | species | sex | birth | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen | cat | m | 1994-03-17 | NULL |
| Buffy | Harold | dog | f | 1989-05-13 | NULL |
+-------+--------+---------+------+------------+-------+
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): In aggregated query without GROUP BY, expression #1 of SELECT list contains nonaggregated column 'menagerie.pet.owner'; this is incompatible with sql_mode=only_full_group_by
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 MySQL Handling of GROUP BY.
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.
| name | date | type | remark |
|---|---|---|---|
| Fluffy | 1995-05-15 | litter | 4 kittens, 3 female, 1 male |
| Buffy | 1993-06-23 | litter | 5 puppies, 2 female, 3 male |
| Buffy | 1994-06-19 | litter | 3 puppies, 3 female |
| Chirpy | 1999-03-21 | vet | needed beak straightened |
| Slim | 1997-08-03 | vet | broken rib |
| Bowser | 1991-10-12 | kennel | |
| Fang | 1991-10-12 | kennel | |
| Fang | 1998-08-28 | birthday | Gave him a new chew toy |
| Claws | 1998-03-17 | birthday | Gave him a new flea collar |
| Whistler | 1998-12-09 | birthday | First birthday |
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,->TIMESTAMPDIFF(YEAR,birth,date) 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
an 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 permits 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.