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
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
TABLE statement for the
table might look like this:
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.
|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|
|Fang||1998-08-28||birthday||Gave him a new chew toy|
|Claws||1998-03-17||birthday||Gave him a new flea collar|
Load the records like this:
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
(YEAR(date)-YEAR(birth)) - (RIGHT(date,5)<RIGHT(birth,5)) AS age,->
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:
FROM clause joins two tables
because the query needs to pull information from both of
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
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
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:
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.