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Saturday, February 15, 2014

Difference between primary key and unique key in SQL Server

In this article we will discuss about what is the difference between primary key and unique key in SQL server.

In my last articles we have discussed about:
- Left outer join and Right outer join in sql server

- constraints in sql server

- Implement delete functionality in Gridview in Asp.Net

Unique key:
-There may be more than one unique key can be present inside a table.

-It supports null values but only once.

-But it doesn't support duplicates like suppose inside column there are two null values then Sql Server internally give some different values but values are not equal.

-Unique key constraint is used to prevent the duplication of key values within the rows of a table and allow null values.

- By default Unique Key will Generate Non-Clustured Index.

Primary key:
-Primary key is(Unique key+not null). Primary Key is a combination of Unique and NOT NULL Constraints so it can’t have duplicate values or any NUll.

-There must be one primary key can be present inside a table.

-It doesn't support null values .

-It doesn't support duplicates.

-Primary key allows each row in a table to be uniquely identified and ensures that no duplicate rows exist.

- By default Primary Key will generate Clustured Index.