Different ways to create a foreign key in an existing table

Here are the 3 different ways to add a Foreign Key in an existing table:-

1. By using ADD CONSTRAINT with update and delete cascade command:-

ALTER TABLE dbo.State

ADD CONSTRAINT fk_city_id FOREIGN KEY (CityID)

       REFERENCES City (CityID)

       ON UPDATE CASCADE

       ON DELETE CASCADE;

In this query update cascade will update the column when referenced column is updated and delete cascade will delete the row when data in the referenced column is deleted.

   2. By using ADD column name along with constraint
     
      Alter table dbo.State

     ADD CityID int CONSTRAINT fk_city_id FOREIGN KEY REFERENCES City (CityID)

    3. By using ADD CONSTRAINT without cascade

     Alter table dbo.State

     ADD CONSTRAINT fk_city_id FOREIGN KEY(CityID) REFERENCES City (CityID)

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