How do I edit my database to not allow duplicate values in a field

[Dauz]

Here's my problem:

There's a field in my database called ESN that currently allows duplicate values. The values are stored as either 8 digit hex values (e.g. 8AFC11FF) or as 11 digit decimal values (12212344311). We now are not allowing any new and existing values in this field to be duplicate.

HOW WOULD I DO THIS?

I'm sort of new to SQL Server. Any help would be great!

Thanks in advance

 

[ClaireHCM]

If your table already exists duplicate key,first you need to delete all the duplicate rows.
After deleting all the duplicate rows.Add primary key on that column where you dont want any duplicate value occurs again.

alter table table_name add constraint PK_name
primary key clustered (column name)

 

[Dauz]

Thanks, I did that and it works great. BUT! I was wondering now how I could notify the user of a duplicate value via the .asp webpage their entering the data from.