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