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 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 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's easiest to load the initial records by
 creating a tab-delimited text file containing the 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,
          -> (YEAR(date)-YEAR(birth)) - (RIGHT(date,5)<RIGHT(birth,5)) AS age,
          -> remark
          -> FROM pet, event
          -> WHERE pet.name = event.name AND 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 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 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, 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 to refer to the
 columns and keep straight which instance of the table each column
 reference is associated with.
 

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