CREATE TABLE Statement This is a more complex example in which a  product_order  table has foreign keys for two other tables. One foreign key references a two-column index in the  product  table. The other references a single- column index in the  customer  table: You can add a foreign key constraint to an existing table using the following  ALTER TABLE  syntax: The foreign key can be self referential (referring to the same table). When you add a foreign key constraint to a table using  ALTER TABLE ,  remember to first create an index on the column(s) referenced by the foreign key. You can drop a foreign key constraint using the following  ALTER TABLE  syntax: ALTER TABLE  tbl_name  DROP FOREIGN KEY  fk_symbol ; If the  FOREIGN KEY  clause defined a  CONSTRAINT  name when you created the constraint, you can refer to that name to drop the foreign key constraint. Otherwise, a constraint name was generated internally, and you must use that value. To determine the foreign key constraint name, use  SHOW CREATE TABLE : 2578 Dropping Foreign Key Constraints Adding Foreign Key Constraints CREATE TABLE product ( category INT NOT NULL, id INT NOT NULL, price DECIMAL, PRIMARY KEY(category, id) ) ENGINE=INNODB; CREATE TABLE customer ( id INT NOT NULL, PRIMARY KEY (id) ) ENGINE=INNODB; CREATE TABLE product_order ( no INT NOT NULL AUTO_INCREMENT, product_category INT NOT NULL, product_id INT NOT NULL, customer_id INT NOT NULL, PRIMARY KEY(no), INDEX (product_category, product_id), INDEX (customer_id), FOREIGN KEY (product_category, product_id) REFERENCES product(category, id) ON UPDATE CASCADE ON DELETE RESTRICT, FOREIGN KEY (customer_id) REFERENCES customer(id) ) ENGINE=INNODB; ALTER TABLE  tbl_name ADD [CONSTRAINT [ symbol ]] FOREIGN KEY [ index_name ] ( col_name , ...) REFERENCES  tbl_name  ( col_name ,...) [ON DELETE  reference_option ] [ON UPDATE  reference_option ] mysql>  SHOW CREATE TABLE child\G *************************** 1. row *************************** Table: child Create Table: CREATE TABLE ‘child‘ ( ‘id‘ int DEFAULT NULL, ‘parent_id‘ int DEFAULT NULL,