Published on: 03 Oct, 2019

How to add a sql foreign key constraint into an existing table in SQL Server?

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A foreign key constraint in SQL Server is used to link two tables together. It refers to the primary key of another table. The main purpose to use constraint is to ensure the integrity of the data between two tables. You will be prompted an error if you try to insert the id of another table that does not exist.

You can run the following SQL query to add a constraint foreign key into an existing table.

ALTER TABLE Table_Name
ADD CONSTRAINT FK_ID_Column_Name
FOREIGN KEY (ID_Column_Name) REFERENCES Table_Name(ID_Column_Name)
In some situations, if you end up need to remove the existing constraint foreign key. You can run the following SQL query.
ALTER TABLE Orders
DROP CONSTRAINT FK_ID_Column_Name

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