Primary Key Constraint And Unique Constraint in SQL Server

Hi Vipendra,

Both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a nonclustered index by default. Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only.



Thanks

Primary Key:
1.Clustered index.only one primary key per table.
2.Doesnt allow null.


Unique key:
1.Non clustered index. you can have as many unique keys in a  table.
2.Allows null