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Difference Between Primary key and Unique key

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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