0| PRIMARY KEY | UNIQUE KEY |
| Primary key is a set of one or more fields/columns of a table that uniquely identify a record in database table. Primary keys must contain unique values. | Unique-key is to prevent duplicate values in a column. prevents two records from having identical values in a column. |
| There will be only one PRIMARY KEY per table. | There may be more that one UNIQUE KEY per table. |
| Should not have a NULL value. | Can have a NULL value. |
| Unique key identifier of the record. | May not be unique. |
| By default, creates clustered index. | Creates non-clustered index. |
| Values can’t change or delete. | Values can change or delete. |
| Can be made into a foreign key into another table. | Can not be made into a foreign key into another table. |
