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Referencing a foreign key to the primary key of the same table
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Referencing a foreign key to the primary key of the same table 
The point in doing this type of referencing is to create a hierarchy among the records and to maintain data consistency.

For eg:- Consider a table which has n number of columns, two among them are user_id and created_user_id.

user_id is the primary key and created_user_id is the foreign key which references the user_id.

By this way of referencing, we cannot delete a user_id record which appears as a created_user_id.

In simple terms if user_id is a child, and created_user_id is a father, You cannot kill a child without killing his father  (or kill a father to kill all of his children ;))
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