Which of the following is NOT a key in Relational Model?

The correct answer is Option 3) Duplicate Key.

Key Points

• In the Relational Model of databases, keys are used to uniquely identify tuples (rows) in a relation (table).
• Primary Key: A unique identifier for each record in a table. No two rows can have the same primary key, and it cannot contain NULLs.
• Foreign Key: A field (or collection of fields) in one table that refers to the primary key in another table. It is used to establish and enforce a link between the data in the two tables.
• Composite Key: A key made up of two or more attributes that together uniquely identify a record.
• Duplicate Key ❌ is not a recognized concept in the relational model. The term contradicts the basic purpose of a key, which is to ensure uniqueness.

Additional Information

• Candidate Key: A set of attributes that can uniquely identify a record. One of them is selected as the primary key.
• Super Key: A set of attributes that uniquely identifies each record in a table. It may contain extra attributes not necessary for uniqueness.
• All keys in relational models are meant to avoid duplication and enforce integrity.

Conclusion: Since the term "Duplicate Key" implies the presence of non-unique identifiers, it is not considered a valid key in the relational model.