Consider the following schema:
$\text{Sailors(sid,sname,rating,age)}$
$\text{Boats(bid,bname,colour)}$
$\text{Reserves(sid,bid,day)}$
Two boats can have the same name but the colour differentiates them.
The two relations
$\rho (Tempsids,(\pi _{sid,bid}\,Reserves)/(\pi_{bid}(\sigma_{bname='Ganga'}\,Boats ))),$
$\pi_{sname}(Tempside\Join Sailors)$
If $/$ is division operation, the above set of relations represents the query
- Names of sailors who have reserved all boats called Ganga.
- Names of sailors who have not reserved any Ganga boat.
- Names of sailors who have reserved at least one Ganga boat.
- Names of sailors who have reserved at most one Ganga boat.
