July2016 cs Q18
0. 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.
