The Unique Constraint unique(A1,A2,....Am) The unique specification states that the attributes A1,A2,...Am form a candidate key. Candidate keys are permitted to be null (in contrast to primary keys). Database System Concepts,5th Edition,Oct 5.2006 4.11 Silberschatz,Korth and SudarshanDatabase System Concepts, 5th Edition, Oct 5. 2006 4.11 ©Silberschatz, Korth and Sudarshan The Unique Constraint unique ( A1 , A2 , …, Am) The unique specification states that the attributes A1, A2, … Am form a candidate key. Candidate keys are permitted to be null (in contrast to primary keys)