为自己加油

(1)R∩S(2)R∪S(3)R-S(4)πA，B(R)??πB，C(S)

.(1)SELECTA，B，CFROMRUNIONSELECTA，B，CFROMS；(2)SELECTA，B，CFROMRINTERSECTSELECTA，B，CFROMS；(3)SELECTA，B，CFROMREXCEPTSELECTA，B，CFROMS；(4)SELECTR.A.R.B，S.CFROM(RNATURALINNERJOINS)WHERER.B=S.B