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(mysql.info)Multiple tables

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Using More Than one Table
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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 pet name so you know which animal each event
     pertains to.

   * A date so 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's easiest to load the initial records by
creating a tab-delimited text file containing the information:

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've learned from the queries you've 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 you want to find out the ages of each pet when they had their
litters.  The `event' table indicates when this occurred, but to
calculate the age of the mother, you need her birth date.  Because that
is stored in the `pet' table, you need both tables for the query:

     mysql> SELECT pet.name, (TO_DAYS(date) - TO_DAYS(birth))/365 AS age, remark
         -> FROM pet, event
         -> WHERE pet.name = event.name AND type = "litter";
     +--------+------+-----------------------------+
     | name   | age  | remark                      |
     +--------+------+-----------------------------+
     | Fluffy | 2.27 | 4 kittens, 3 female, 1 male |
     | Buffy  | 4.12 | 5 puppies, 2 female, 3 male |
     | Buffy  | 5.10 | 3 puppies, 3 female         |
     +--------+------+-----------------------------+

There are several things to note about this query:

   * The `FROM' clause lists 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 `WHERE' clause to match up records in the
     two tables based on the `name' values.

   * 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 pair up males and females of like species:

     mysql> SELECT p1.name, p1.sex, p2.name, p2.sex, p1.species
         -> FROM pet AS p1, pet AS p2
         -> WHERE 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 in order to refer
to the columns and keep straight which instance of the table each
column reference is associated with.