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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:
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'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 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 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:
FROM
clause lists two tables because the query needs to pull
information from both of them.
name
column. The query uses
WHERE
clause to match up records in the two tables based on the
name
values.
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 in order to refer to the columns and keep straight which instance of the table each column reference is associated with.
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