--测试数据如下
create table tbla
(
    typename varchar(50)
)
insert into tbla
select 'dt-051' union all
select 'dt-052' union all
select 'dt-053' union all
select 'dt-054' union all
select 'dt-055' union all
select 'dt-056'


create table tblb
(
    typename varchar(50),
    num int,
)
insert into tblb
select 'dt-056',30 union all
select 'dt-056',20 union all
select 'dt-051',10 union all
select 'dt-052',30 union all
select 'dt-053',20 union all
select 'dt-051',30


create table tblc
(
    typename varchar(50),
    num int,
)
insert into tblc
select 'dt-056',30 union all
select 'dt-055',20 union all
select 'dt-056',20 union all
select 'dt-055',30 union all
select 'dt-051',50 union all
select 'dt-053',10

select a.typename as 型号,sum(isnull(b.num,0)) as 共领料 
from tbla as a 
left join tblb as b on b.typename = a.typename 
group by a.typename
order by a.typename asc
--结果没有问题
/*
dt-051    40
dt-052    30
dt-053    20
dt-054    0
dt-055    0
dt-056    50
*/

select a.typename as 型号,sum(isnull(c.num,0)) as 共入库 
from tbla as a 
left join tblc as c on c.typename = a.typename 
group by a.typename
order by a.typename asc

--结果没有问题
/*
dt-051    50
dt-052    0
dt-053    10
dt-054    0
dt-055    50
dt-056    50
*/

--但是
select a.typename as 型号,sum(isnull(b.num,0)) as 共领料,sum(isnull(c.num,0)) as 共入库 
from tbla as a 
left join tblb as b on b.typename = a.typename 
left join tblc as c on c.typename = a.typename 
group by a.typename,b.typename,c.typename
order by a.typename asc

--结果就出现问题了
/*
dt-051    40    100
dt-052    30    0
dt-053    20    10
dt-054    0    0
dt-055    0    50
dt-056    100    100
*/

--正常结果如下
/*
dt-051    40    50
dt-052    30    0
dt-053    20    10
dt-054    0    0
dt-055    0    50
dt-056    50    50
*/

--问题出在哪

--使用如下的sql语句 结果就是正确的 区别在哪求解释
select a.typename,isnull(b.sumnum,0) as 共领料,isnull(c.sumnum,0) as 共入库
from tbla as a
left join (select typename,sum(num) as sumnum from tblb group by typename)
b on b.typename = a.typename
left join (select typename,sum(num) as sumnum from tblc  group by typename)
c on c.typename = a.typename
order by a.typename asc
--结果
/*
dt-051    40    50
dt-052    30    0
dt-053    20    10
dt-054    0    0
dt-055    0    50
dt-056    50    50
*/

--看如下sql
select a.typename as 型号,isnull(b.num,0) as 共领料
,isnull(c.num,0) as 共入库 
from tbla as a 
left join tblb as b on b.typename = a.typename 
left join tblc as c on c.typename = b.typename 
--结果集如下
型号                                                 共领料         共入库
-------------------------------------------------- ----------- -----------
dt-051                                             10          50
dt-051                                             30          50
dt-052                                             30          0
dt-053                                             20          10
dt-054                                             0           0
dt-055                                             0           0
dt-056                                             30          30
dt-056                                             30          20
dt-056                                             20          30
dt-056                                             20          20

(10 row(s) affected)
--此时你在用sum,那么它的操作时累加的所以会出现这个问题

------解决方案--------------------

提供另一种写法,
SQL code

create table tbla
( typename varchar(50))

insert into tbla
select 'dt-051' union all
select