Types Of Constraints-SQL Server

1.Primary Key
2.Foreign key or references integirity
3.Unique
4.Check
5.NOt null
6.Default

The unique key allows only one null values.But the primary key does,nt allows a null values
By default The primary key is clustered index.
By default the unique key is NOn Clustered index.

Clustered Index:
It alters the way in which the data is srored in the database.

Non Clustered Index:
It does not alters the way in which the data is stored in the database.

Whenever We are implementing an indexes it uses non clustered index of sorting the data