enforcing with unique indexes..

Hi ,

can anyone tell me how can i enforce below in a table.
I want that no more that one distinct userid exists for a given group_id
in the table.

ie i want 1 to 1 mapping between group_id and userid so that , there shud not be a
single group_id having more that one kind of userid.

SELECT  group_id  from eyp_listing group by group_id  having  count(distinct userid) > 1  ;

always returns empty.

can it be done with some sort of UNIQUE INDEX?


Regds
MAllah.





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