What Is Primary Key?
The primary key is the minimum set of traits that distinguishes any row of a table. It cannot have NULL and duplicate values. The primary key is used to add integrity to the table.
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In the case of a primary key, both Duplicate and NULL values are not valid. And, it can be utilized as foreign keys for different tables.
What Is a Unique Key?
A unique Key is an individual value that is used to protect duplicate values in a column. The foremost purpose of a unique key in a table is to prevent duplicate values. However, when it comes to a unique value, the primary key also includes it. So, there is one big difference that makes a unique key different, and it is: a unique key can have a NULL value which is not supported in a primary key.
Difference Between Primary Key and Unique Key
| Parameters | Primary Key | Unique Key |
| Basic | The primary key is accepted as a unique or sole identifier for every record in the table. | When the primary key is not present in the table, then the unique key is also used as a unique identifier for records |
| NULL | In the case of a primary key, we cannot save NULL values. | In the case of a unique key, we can save a null value, however, only one NULL value is supported. |
| Purpose | It supports entity integrity. | It supports unique data. |
| Index | The primary key tends to generate a clustered index by default. | The unique key tends to generate a non-clustered index. |
| Number of Key | Each table holds just one primary key. | A table can hold more than one unique key. |
| Value Modification | With the primary key, we cannot modify or delete the values. | With the unique key, we can modify the column values. |
| Uses | It is used to recognize specific records in the table. | It anticipates storing duplicate entries in a column except for a NULL value. |
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