Primary key must be single Unique key can be more than one

Primary key does not allow null value. Unique key constraints allow one null value.

8 down vote A PRIMARY Key and UNIQUE Key constraints both are similar and it provide unique enforce uniqueness of the column on which they are defined.Primary KeyPrimary key cannot have a NULL value. Each table can have only one primary key. By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index. Primary key can be related with another table's as a Foreign Key. We can generated ID automatically with the help of Auto Increment field. Primary key supports Auto Increment value. Unique KeyUnique Constraint may have a NULL value. Each table can have more than one Unique Constraint. By default, Unique key is a unique non-clustered index. Unique Constraint can not be related with another table's as a Foreign Key. Unique Constraint doesn't supports Auto Increment value.