Question 29Asked in UGC NET Computer Science, June 2016 (Paper II & III, Selected Questions)
In a relational database model, NULL values can be used for all but which one of the following?
ATo allow duplicate tuples in the table by filling the primary key column(s) with NULL.
BTo avoid confusion with actual legitimate data values like 0 (zero) for integer columns and '' (the empty string) for string columns.
CTo leave columns in a tuple marked as "unknown" when the actual value is unknown.
DTo fill a column in a tuple when that column does not really "exist" for that particular tuple.
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Correct answer: (A) To allow duplicate tuples in the table by filling the primary key column(s) with NULL.
Explanation
The SQL null is the term used to represent a missing value. A null value in a table is a value in a field that appears to be blank. A field with a null value is a field with no value.
From UGC NET Computer Science PYQ, June 2016 (Paper II & III, Selected Questions)