apply unique constraint in MS

I want to apply a unique constraint to an existing field in the database.
The constraints dialog box asks for a "constraint expression". I have went
through bol and google without finding a solution. I know how to create a
column with a unique constraint but no where can I find how to write the
"expression" to make an existing column unique in Management Studio.
How?
Thanks,
TCREATE TABLE Banana (Peal char(5) NOT NULL);
ALTER TABLE Banana
ADD Constraint Demonstrate UNIQUE(Peal);
INSERT Banana values('a')
INSERT Banana values('b')
INSERT Banana values('b')
(1 row(s) affected)
(1 row(s) affected)
Server: Msg 2627, Level 14, State 1, Line 3
Violation of UNIQUE KEY constraint 'Demonstrate'. Cannot insert
duplicate key in object 'dbo.Banana'.
The statement has been terminated.
Roy Harvey
Beacon Falls, CT
On Tue, 13 Nov 2007 17:06:59 -0700, "Tina"
<TinaMSeaburn@.nospamexcite.com> wrote:
>I want to apply a unique constraint to an existing field in the database.
>The constraints dialog box asks for a "constraint expression". I have went
>through bol and google without finding a solution. I know how to create a
>column with a unique constraint but no where can I find how to write the
>"expression" to make an existing column unique in Management Studio.
>How?
>Thanks,
>T
>