In MySQL, how do I create a column with a foreign key?

开发者_StackOverflowA column with a foreign key to another table's column. How do I do that?

This is not possbile with table engine MyISAM but with InnoDB, e.g.:

CREATE TABLE parent (id INT NOT NULL,
                     PRIMARY KEY (id)
) ENGINE=INNODB;
CREATE TABLE child (id INT, parent_id INT,
                    INDEX par_ind (parent_id),
                    FOREIGN KEY (parent_id) REFERENCES parent(id)
                      ON DELETE CASCADE
) ENGINE=INNODB;

Otherwise (with MyISAM) you just have to check the columns manually. It is still (at least logical) a foreign key but without the constraint.

In the end a foreign key is just a reference to another table. The table does not necessarily have to know about this, but it makes life easier.

First, make sure you are using InnoDB (or another engine that supports foreign keys; MyISAM does not). Then, use the appropriate DDL statements.