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UGC-NET | UGC NET CS 2016 Aug – II | Question 18
  • Last Updated : 11 Apr, 2018

Consider the following statements regarding relational database model:
(a) NULL values can be used to opt a tuple out of enforcement of a foreign key.
(b) Suppose that table T has only one candidate key. If Q is in 3NF, then it is also in BCNF.
(c) The difference between the project operator (Π) in relational algebra and the SELECT keyword in SQL is that if the resulting table/set has more than one occurrences of the same tuple, then Π will return only one of them, while SQL SELECT will return all.
One can determine that:
(A) (a) and (b) are true.
(B) (a) and (c) are true.
(C) (b) and (c) are true.
(D) (a), (b) and (c) are true.


Answer: (D)

Explanation: IN relational database model:
(a) NULL values can be used to opt a tuple out of enforcement of a foreign key.Correct
(b) Suppose that table T has only one candidate key. If Q is in 3NF, then it is also in BCNF.Correct
(c) The difference between the project operator (Π) in relational algebra and the SELECT keyword in SQL is that if the resulting table/set has more than one occurrences of the same tuple, then Π will return only one of them, while SQL SELECT will return all.Correct
So, option (D) is correct.

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