或者,您可以利用使用子查询的优势:
select A.ID, A.Total, b.SB as AmountB, c.SC as AmountC, d.SD as AmountD
from A
inner join (select ExtID, sum(Amount) as SB from B group by ExtID) b on A.ID = b.ExtID
inner join (select ExtID, sum(Amount) as SC from C group by ExtID) c on c.ExtID = A.ID
inner join (select ExtID, sum(Amount) as SD from D group by ExtID) d on d.ExtID = A.ID