Difference between Primary Key and Unique Key?

In this post, I will explain the difference between Primary Key and Unique Key.

Primary Key and Unique Key provide uniqueness for a column or set of columns in a table. But there are different characteristics or behaviour of them, which makes Primary Key and Unique Key different. Please find below the difference between Primary Key and Unique Key :

Sr No	Primary Key	Unique Key
1	A database table can have only one Primary Key.	A database table can have more than one Unique Key.
2	Primary Key is not nullable (Doesn't allow NULL value).	Unique Key is nullable (Allows at least one NULL value).
3	Primary Key generates a Unique Clustered Index.	Unique Key generates a Unique Non-Clustered Index.
4	Primary Key is used to identify a row (a record) in a table.	Unique Key is used to prevent duplicate values in a column.