What is the Difference Between Primary key and Unique key?

The primary key and unique key are both used to ensure the uniqueness of data in a table, but they serve different purposes and have distinct characteristics:

1. Purpose: The primary key is used to uniquely identify each row in a table and enforce entity integrity, while the unique key is used to prevent duplicate values in a column and enforce unique data.
2. Existence: A table can have only one primary key, but it can have multiple unique keys.
3. Nullability: Primary keys cannot accept NULL values, while unique keys can accept multiple NULL values.
4. Index: A primary key supports a clustered index by default, whereas a unique key supports a non-clustered index.
5. Modification: You cannot change or delete values stored in primary keys, but you can modify unique key values.

In summary, a primary key is used to identify each row uniquely and enforce entity integrity, while a unique key is used to prevent duplicate values in a column and enforce unique data. A table can have only one primary key, but it can have multiple unique keys. Primary keys cannot accept NULL values, while unique keys can. A primary key supports a clustered index by default, and a unique key supports a non-clustered index. You cannot change or delete values stored in primary keys, but you can modify unique key values[

Comparative Table: Primary key vs Unique key

Here is a table comparing the differences between primary keys and unique keys:

Both primary and unique keys are used to ensure that each row in a table has a unique identifier. However, primary keys do not allow NULL values, while unique keys can accept one NULL value in a column. A table can have only one primary key, but it can have multiple unique keys.