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Aug2016 cs Q18

0. 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:

  • Option : 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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