Consider the following statements S1 and S2 about the relational data model:S1: A relation scheme ca

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GATE-CS is a competitive examination conducted in India. This question is from the 2021 GATE-CS Database Management Systems paper.

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This GATE-CS 2021 Database Management Systems question has a medium difficulty level and is marked as an important question.

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Question

Consider the following statements S1 and S2 about the relational data model:S1: A relation scheme can have at most one foreign key.S2: A foreign key in a relation scheme R cannot be used to refer to tuples of R itself.Which one is correct?

Accepted Answer

Correct Answer: (C) Both S1 and S2 are falseConcept -- Foreign Keys in Relational Databases (from Ramakrishnan and Gehrke, Database Management Systems; Silberschatz, Korth, Sudarshan):S1 is FALSE: A relation scheme can have multiple foreign keys. For example, an ORDER table might have a foreign key referencing the CUSTOMER table and another foreign key referencing the PRODUCT table. The relational model places no limit on the number of foreign keys a relation can have.S2 is FALSE: A foreign key in a relation scheme R CAN refer to tuples within R itself. This is called a self-referential foreign key. A classic example is an EMPLOYEE table where the 'manager' attribute is a foreign key that references the 'empId' attribute in the same EMPLOYEE table (a manager is also an employee). SQL and the relational model both support self-referential foreign keys explicitly.

Consider the following statements S1 and S2 about the relational data model:S1: A relation scheme can have at most one foreign key.S2: A foreign key in a relation scheme R cannot be used to refer to tuples of R itself.Which one is correct?

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Consider the following statements S1 and S2 about the relational data model:S1: A relation scheme can have at most one foreign key.S2: A foreign key in a relation scheme R cannot be used to refer to tuples of R itself.Which one is correct?

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