retriving customers with perticular usergroup is null

[SwapnaliP]

Hi everyone,
Please help to write a flexible search query to retrieve customers who does not have usergroup of type lets say "X".

I wrote a query to retrive all customers and their attached principleGroups but I am not sure how can I check type X is not present for those customers.

select {m.uid},{p.uid} FROM { PrincipalGroupRelation as pg JOIN PrincipalGroup as p ON {pg.target} = {p.pk} JOIN Customer as m ON {pg.source} = {m.pk} } order by {m.uid}

I want customers who does not X groupid uid attached to them.
Please help.