# UGC-NET | UGC NET CS 2016 Aug – II | Question 18

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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