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How can we add a FOREIGN KEY constraint to the field of an existing MySQL table?
We can add a FOREIGN KEY constraint to a column of an existing MySQL table with the help of ALTER TABLE statement.
Syntax
ALTER TABLE table_name ADD FOREIGN KEY (colum_name) REFERENCES table having Primary Key(column_name);
Example
Suppose we want to add a FOREIGN KEY constraint on the table ‘Orders1’ referencing to the table ‘Customer’ which have column ‘Cust_Id’ as the Primary Key. It 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)
Updated on: 30-Jul-2019
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