Hvordan fjerner man null-rækker fra sql-forespørgselsresultatet?

select 
  c.Id, c.FirstName, dbo.GetClientStaffContacts(c.Id, '27,31') as Staff 
from 
  Client c 
where
  dbo.GetClientStaffContacts(c.Id, '27,31') is not null
order 
  by c.Id