The following table will provide us the differences between PRIMARY KEY and UNIQUE constraint −
| PRIMARY KEY | UNIQUE Constraint |
|---|---|
| 1. Only one Primary key can be created on a table. | 1. More than one UNIQUE Constraints can be added to a table. |
| 2. Primary key creates clustered index by default. | 2. UNIQUE Constraint creates a non-clustered index by default. |
| 3. We cannot insert null values in the column which is defined as PRIMARY KEY. | 3. We can insert null values in the column having a UNIQUE constraint. |