DEPNO | DNAME | LOC
--- | --- | ---
20 | RESEARCH | DALLAS
30 | SALES | NEW
40 | MARKETING | BOSTON
- | UNIQUE Key Constraint (no row may duplicate a value in the constraint's column) | -

DEPNO | DNAME | LOC | Comments
--- | --- | --- | ---
50 | SALES | NEW YORK | This row violates the UNIQUE key constraint, because "SALES" is already present in another row; therefore, it is not allowed in the table.
60 | N/A | BOSTON | This row is allowed because a null value is entered for the DNAME column; however, if a NOT NULL constraint is alos defined on the DNAME column, this row is not allowed.