How to restric the duplicate value on nvarchar column in sql

Question

Hi,

How to restric the duplicate value on nvarchar column in sql Express 2008

Below is my table

Hide Copy Code

CREATE TABLE [dbo].[User](

[User_No] [int] IDENTITY(1,1) NOT NULL,

[User_Name] [nvarchar](max) NOT NULL

) ON [PRIMARY]

GO

I want to restric if the User_Name is duplicate.