alter the field constraint to a table

you cant add any unique columns after the table having more then 2 records because unique constraint allow one null value and never allow duplicate
if u try this bellow query

alter table sales add snnno int unique

u will get the below error

Server: Msg 1505, Level 16, State 1, Line 1
CREATE UNIQUE INDEX terminated because a duplicate key was found for index ID 43. Most significant primary key is '<NULL>'.
Server: Msg 1750, Level 16, State 1, Line 1
Could not create constraint. See previous errors.
The statement has been terminated.

so u create one child table for this columns
may be use normalization way u will get solution

try like this

ALTER TABLE MyTable
ADD AddDate smalldatetime NULL
CONSTRAINT AddDateDflt
DEFAULT getdate() WITH VALUES

Also see this Link[^]

Thanks
--RA