We can add FOREIGN KEY constraints to columns of existing MySQL tables with the help of ALTER TABLE statement.
ALTER TABLE table_name ADD FOREIGN KEY (colum_name) REFERENCES table with Primary Key(column_name);
Suppose we want to add a foreign key constraint on the table "Orders1", referencing the table "Customer", the primary key of the table is the "Cust_Id" column. This can be done with the help of the following query -
mysql> Alter table orders1 add FOREIGN KEY(Cust_id) REFERENCES Customer(Cust_id); Query OK, 0 rows affected (0.21 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> Describe ORDERS1; +--------------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------------+-------------+------+-----+---------+-------+ | order_id | int(11) | NO | PRI | NULL | | | Product_name | varchar(25) | YES | | NULL | | | orderdate | date | YES | | NULL | | | Cust_id | int(11) | YES | MUL | NULL | | +--------------+-------------+------+-----+---------+-------+ 4 rows in set (0.05 sec)
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