What is the difference between unique key and primary key?

Answer:

 

This is a typical SQL Server interview question and below is the comparison sheet.

Unique Key	Primary Key
Unique key can have nulls	Primary key cannot have nulls.
In a single table we can create multiple unique keys.	In a single table we can have only one primary key.
It creates a non-clustered index by default.	It created a clustered index by default.
Both unique keys and primary keys can be referenced by foreign key.