220.127.116.11 Using More Than one Table
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.
|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:
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:
FROMclause 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
namecolumn. The query uses an
ONclause to match up records in the two tables based on the
The query uses an
INNER JOINto combine the tables. An
INNER JOINpermits rows from either table to appear in the result if and only if both tables meet the conditions specified in the
ONclause. In this example, the
ONclause specifies that the
namecolumn in the
pettable must match the
namecolumn in the
eventtable. If a name appears in one table but not the other, the row will not appear in the result because the condition in the
namecolumn 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 live 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 p1.death IS NULL AND p2.sex = 'm' AND p2.death IS NULL; +--------+------+-------+------+---------+ | name | sex | name | sex | species | +--------+------+-------+------+---------+ | Fluffy | f | Claws | m | cat | | Buffy | f | Fang | 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.