What is UNIQUE Constraint in SQL Server?
In this article i will explain what is UNIQUE Constraint in SQL Server.
In previous articles i explained what is Constraint, how many Constraints available in SQL Server, and what is NOT NULL, CHECK Constraint in SQL Server. Now i will explain what is UNIQUE Constraint in SQL Server.
UNIQUE constraint enforce the uniqueness of the values in a set of columns. In a UNIQUE constraint, no two rows in the table can have the same value for the columns.
We can create UNIQUE constraint in two ways:
- Column Level
- Table Level
Now i'm creating Employee table where EmpNo column is UNIQUE.
Create table Employee ( EmpNo int unique, -- column level EmpName varchar(50) )
Now I'm creating Employee table where EmpNo column is UNIQUE.
Create table Employee ( EmpNo int, EmpName varchar(50), constraint UK_EmpNo unique(EmpNo) -- table level )
Note: UNIQUE allow one null value, but primary keys do not allow for NULL as one of the unique values.
CreatedOct 23, 2013
UpdatedOct 03, 2020