UGC-NET | UGC NET CS 2016 July – II | Question 18

In a relational database model, NULL values can be used for all but which one of the following?
(A) To allow duplicate tuples in the table by filling the primary key column(s) with NULL.
(B) To avoid confusion with actual legitimate data values like 0 (zero) for integer columns and ’’ (the empty string) for string columns.
(C) To leave columns in a tuple marked as ’’unknown’’ when the actual value is unknown.
(D) To fill a column in a tuple when that column does not really ”exist” for that particular tuple.


Answer: (A)

Explanation:

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