sql多表查询优化
TableA(A1,A2)
TableB(B1,B2)
TableAB(id,A1,B1)
TableA和TableB是多对多关系，通过中间表TableAB连接
TableC(C1,A1,AmountC)
TableD(D1,A1,AmountD)
TableC和TableD通过A1字段和TableA连接,是多对一关系
第一种方法
select A1,A2
from TableA
left join
(
select B1,B2,A1
from TableB
left join TableAB
on TableAB.B1 = TableB.B1
) TableBNew onTableBNew.A1 = TableA.A1
left join
(
select C1,sum(AmountC),A1
from TableC
group by A1
) TableCNew on TableCNew.A1 = TableA.A1
left join
(
select D1,sum(AmountD),A1
from TableD
group by A1
) TableDNew on TableDNew.A1 = TableA.A1
第二种方法
selectTableANew2.A1,TableANew2.A2,TableANew2.AmountC,TableANew2.AmountD,TableB.B1,TableB.B2
from
(
selectTableANew.A1,TableANew.A2,TableANew.AmountC,sum(TableD.AmountD)
from
(
select A1,A2,sum(TableC.AmountC)AmountC
from TableA
left join TableC
onTableC.A1 = TableA.A1
group by TableA.A1
) TableANew
left join TableD
onTableD.A1 = TableANew.A1
group by TableANew.A1
)TableANew2
left join TableAB
on TableAB.A1 = TableANew2.A1
left join TableB
on TableB.B1 = TableAB.B1
第一种方式通过子查询，索引很难发挥作用
第二种方式表逐层嵌入查询，通过对比测试，如果配合索引的话，比第一种方式快一个数量级以上。
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