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UGC NET CSE | June 2016 | Part 2 | Question: 18

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In a relational database model, NULL values canbe used for all but which one of the following?

  1. To allow duplicate tuples in the table by filling the primary key column(s) with NULL
  2. To avoid confusion with actual legitimate data values like 0 (zero) or integer columns and " (the empty string) for string columns
  3. To leave columns in a tuple marked as "unknown" when the actual value is unknown
  4. To fill a column in a tuple when that column does not really  "exist" for that particular tuple
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Null values can not be used in primary key (Entity constraint)
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