Using Foreign Keys

Using Foreign Keys
In MySQL, InnoDB tables support checking of foreign key constraints. See Section 14.2, “The InnoDB
Storage Engine”, and Section 1.8.5.4, “Foreign Key Differences”.
A foreign key constraint is not required merely to join two tables. For storage engines other than
InnoDB, it is possible when defining a column to use a REFERENCES tbl_name(col_name) clause,
which has no actual effect, and serves only as a memo or comment to you that the column which
you are currently defining is intended to refer to a column in another table. It is extremely important to
realize when using this syntax that:
• MySQL does not perform any sort of CHECK to make sure that col_name actually exists in
tbl_name (or even that tbl_name itself exists).
• MySQL does not perform any sort of action on tbl_name such as deleting rows in response to
actions taken on rows in the table which you are defining; in other words, this syntax induces no
ON DELETE or ON UPDATE behavior whatsoever. (Although you can write an ON DELETE or ON
UPDATE clause as part of the REFERENCES clause, it is also ignored.)
• This syntax creates a column; it does not create any sort of index or key.
You can use a column so created as a join column, as shown here:
CREATE TABLE person (
id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
name CHAR(60) NOT NULL,
PRIMARY KEY (id)
);
CREATE TABLE shirt (
id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
style ENUM('t-shirt', 'polo', 'dress') NOT NULL,
color ENUM('red', 'blue', 'orange', 'white', 'black') NOT NULL,
owner SMALLINT UNSIGNED NOT NULL REFERENCES person(id),
PRIMARY KEY (id)
);
INSERT INTO person VALUES (NULL, 'Antonio Paz');
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