| It is a unique identifier for each row of a table. | It is a unique identifier for table ROWS in the absence of a Primary key. |
| A single Primary key is ALLOWED in a table. | More than one Unique Key is allowed in a table. |
| NULL VALUE or duplicate value is not permitted. | It can have a single NULL value but a duplicate value is not permitted. |
| When we define a Primary key, a clustered index is automatically created if it does not already exist on the table. | When we define a Unique key, a non-clustered index is created by default to enforce a UNIQUE constraint. |
