Primary & Foreignkey constraints
PRIMARY AND FOREIGN KEY CONSTRAINTS.
Primary key uniquely identifies each record in a table. It must have unique values and cannot contain nulls. In the below example the ROLL_NO field is marked as primary key, that means the ROLL_NO field cannot have duplicate and null values.
Foreign keys are the columns of a table that points to the primary key of another table. They act as a cross-reference between tables.
Introduction to databases by C.J. DATE
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