Q.Explain the difference between a primary key and a foreign key with a suitable example.

Both are keys, but they do opposite jobs — one identifies, the other links. This question asks you to contrast them.

A primary key and a foreign key are both keys, but they do different jobs.

A primary key is an attribute whose value uniquely identifies each tuple within its own relation. It must be unique and never blank.

A foreign key is an attribute in one relation whose values refer to the primary key of another relation. It is the mechanism that creates a relationship (link) between two tables.

Example: Suppose we have two tables:

• Student(RollNo, Name, Class) — here RollNo is the primary key.
• Marks(RollNo, SubjectCode, Marks) — here the RollNo attribute is a foreign key that refers back to RollNo in the Student table.

This foreign key links each marks record to the correct student and ensures that marks cannot be recorded for a student who does not exist (referential integrity). …

Students often think a foreign key is a special new field — in fact it is an ordinary attribute whose special role is that its values must match a primary key in another …