Database : Difference between Primary Key and Unique Key

In  SQL Server, we have two keys which distinctively or uniquely identify a record in the database. Both the keys seems identical, but actually both are different in features and behaviours. In this article, I would like to share the key difference between primary key and unique key. For more help about keys in SQL Server refer the article Different Types of SQL Keys.

Difference between Primary Key & Unique Key

Primary Key

Unique Key

Primary Key can't accept null values.

Unique key can accept only one null value.

By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index.

By default, Unique key is a unique non-clustered index.

We can have only one Primary key in a table.

We can have more than one unique key in a table.

Primary key can be made foreign key into another table.

In SQL Server, Unique key can be made foreign key into another table.

Define Primary key and Unique key

 CREATE TABLE Employee 
 (
 EmpID int PRIMARY KEY, --define primary key
 Name varchar (50) NOT NULL,
 MobileNo int UNIQUE, --define unique key
 Salary int NULL
 )