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.

--
Rajesh Kumar Mallah,
Project Manager (Development)
Infocom Network Limited, New Delhi
phone: +91(11)6152172 (221) (L) ,9811255597 (M)

Visit http://www.trade-india.com ,
India's Leading B2B eMarketplace.