| Primary key | Unique key |
| It cannot have a NULL value. | It may have a NULL value. |
| Each table can have only one primary key. | Each table can have more than one Unique constraint. |
| By default it is organized in the sequence of the clustered index. | By default it is a unique non-clustered index. |
| It supports auto increment value. | It does not support the auto increment value. |
