Description of the illustration adfns039.eps

DEPNO   DNAME       LOC
-----   ---------   --------
20      RESEARCH    DALLAS
30      SALES       NEW YORK
40      MARKETING   BOSTON
-       UNIQUE      -

UNIQUE is a constraint that means no row can duplicate a value in the constraint's column.)

DEPNO   DNAME   LOC        COMMENTS
-----   -----   --------   --------
50      SALES   NEW YORK   1
60      N/A     BOSTON     2

  1. This row violates the UNIQUE key constraint, because SALES is already present in another row; therefore, it is not allowed in the table.

  2. This row is allowed because a NULL value is entered for the DNAME column; however, if a NOT NULL constraint is defined on the DNAME column, this row is not allowed.