Consider the following statement regarding relational database model :
Consider the following statement regarding relational database model :
(1) NULL values can be used to opt a tuple out enforcement of a foreign key
(2) Suppose that table T has only one candidate key. If Q is in 3NF, then it is also in BCNF
(3) 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 :
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Correct answer: (D) (1), (2) and (3) are true
Explanation
(1) NULL values can be used to opt a tuple out of enforcement of a foreign key: correct. (2) If a table has only one candidate key and is in 3NF, then it is also in BCNF: correct. (3) Projection (Π) in relational algebra eliminates duplicate tuples while SQL SELECT returns all of them: correct. So (1), (2) and (3) are true.