主题：多表汇总问题

SELECT  Max(A.型号)  As  型号,Sum(A.数量)  As  进货_数量,Avg(A.单价) As  进货_单价,Sum(A.数量*A.单价)  As  进货_金额,iif(Sum(B.数量) is Null,0,Sum(B.数量))  As  销售_数量,iif(Avg(B.售价) is Null,0,Avg(B.售价)) As  销售_售价,iif(Sum(B.数量*B.售价) is Null,0,Sum(B.数量*B.售价)) As  销售_金额, iif(Sum(C.数量) is Null,0,Sum(C.数量) ) As 退货_数量,iif(Avg(C.单价) is null,0,Avg(C.单价)) As  退货_单价,iif(Sum(C.数量*C.单价) is null,0,Sum(C.数量*C.单价)) As  退货_金额,iif(Sum(A.数量) is null,0,Sum(A.数量)) + iif(Sum(C.数量) is null,0,Sum(C.数量))- iif(Sum(B.数量) is null,0,Sum(B.数量))  As 库存_数量,Avg(A.单价) As  库存_单价,(iif(Sum(A.数量) is null,0,Sum(A.数量)) + iif(Sum(C.数量) is null,0,Sum(C.数量))- iif(Sum(B.数量) is null,0,Sum(B.数量)))* Avg(A.单价) As 库存_金额 FROM ({进货单} A LEFT JOIN {销售单} B ON A.型号= B.型号) LEFT JOIN {退货单} C ON A.型号= C.型号 Group by A.型号