What is the difference between Primary Key and unique key constraint?

Question

What is the difference between Primary key And unique Key constraint?

What\'s the use of it??

Answer 1

Primary key:

1. Primary key is nothing but it uniquely identifies each row in a table.

2. Primary key does not allow duplicate values, nor NULL.

3. Primary key by default is a clustered index.

4. A table can have only one primary key.

Unique Key:

1. Unique key is nothing but it uniquely identifies each row in a table.

2. Unique key does not allow duplicate values, but it allows (at most one) NULL.

3. Unique key by default is a non-clustered index.

This is a fruit full link to understand the Primary Key Database Keys. Keep in mind we have only one clustered index in a table [Talking about SQL Server 2005]. Now if we want to add another unique column then we will use Unique Key column, because Unique Key column can be added more than one.