Multiple-column foreign key in MySQL?

Question

I have a table that has a primary key consisting of two columns (product_id, attribute_id). I have another table that needs to reference this table. How can I make a foreign key in the other table to link it to a row in the table with two primary keys?

Answer 1

There can only be one primary key on a table. The fact in can consist of more than one field does not increase number of primary keys, there's still one.

Since a part of the PK pair is not unique, you obviously have to create a foreign key that refers to two fields as well: REFERENCES t1 (f1, f2).

Answer 2

Something like this ought to do it:

CREATE TABLE MyReferencingTable AS (
   [COLUMN DEFINITIONS]
   refcol1 INT NOT NULL,
   rofcol2 INT NOT NULL,
   CONSTRAINT fk_mrt_ot FOREIGN KEY (refcol1, refcol2)
                        REFERENCES OtherTable(col1, col2)
) ENGINE=InnoDB;

• MySQL requires foreign keys to be indexed, hence the index on the referencing columns
• Use of the constraint syntax enables you to name a constraint, making it easier to alter and drop at a later time if needed.
• InnoDB enforces foreign keys, MyISAM does not. (The syntax is parsed but ignored)