Which of the following is not a constraint

Candidate keys are keys which can be primary key and also able to uniquely identify any row in table. In simply terms you may have couple of Candidate keys and you have choose one of them as primary key. This selection part is the most important skill in database design. Since only primary key can have clustered index in table while unique keys can have Nonclustered index, its important to choose right column or collection of columns as primary key.