Add a foreign key constraint on the existing table – SQL Server

While it is best to create and build the link between tables before the data flow begins, new tables or other existing tables may need to be linked due to a change in plans.

In such scenarios using ALTER statement, foreign keys can be created as shown below.

ALTER TABLE tbOrderDetails
ADD CONSTRAINT FK_OrderID_tbOrderDetails FOREIGN KEY (OrderID)
REFERENCES tbOrders (OrderID)
ON DELETE CASCADE
ON UPDATE CASCADE
;