if object_id('[A]') is not null drop table [A]
go
create table [A]([clientID] varchar(1),[Indate] datetime,[Inresume] int)
insert [A]
select 'a','2008-10-10',2000 union all
select 'a','2018-10-10',2010 union all
select 'b','2028-10-10',2020 union all
select 'b','2038-10-10',2030 union all
select 'c','2048-10-10',2040
go
if object_id('[B]') is not null drop table [B]
go
create table [B]([clientID] varchar(1),[Indate] datetime,[Outresume] int)
insert [B]
select 'a','2008-12-10',1000 union all
select 'a','2018-12-10',1010 union all
select 'b','2028-12-10',1020 union all
select 'b','2038-12-10',1030 union all
select 'c','2048-12-10',1040
go

select clientid,sum(Inresume) as Leftresume
from
(
  select clientid,Inresume from a
  union all
  select clientid,-Outresume from b
) t
group by clientid

/**
clientid Leftresume
-------- -----------
a        2000
b        2000
c        1000

(3 行受影响)
**/

------解决方案--------------------

SQL code

--> 测试数据：[A1]
if object_id('[A1]') is not null drop table [A1]
create table [A1]([clientID] varchar(1),[Indate] datetime,[Inresume] int)
insert [A1]
select 'a','2008-10-10',2000 union all
select 'a','2018-10-10',2010 union all
select 'b','2028-10-10',2020 union all
select 'b','2038-10-10',2030 union all
select 'c','2048-10-10',2040
--> 测试数据：[B2]
if object_id('[B2]') is not null drop table [B2]
create table [B2]([clientID] varchar(1),[Indate] datetime,[Outresume] int)
insert [B2]
select 'a','2008-12-10',1000 union all
select 'a','2018-12-10',1010 union all
select 'b','2028-12-10',1020 union all
select 'b','2038-12-10',1030 union all
select 'c','2048-12-10',1040

;with t
as(
select [clientID],[Inresume] from [A1]
union all
select [clientID],-[Outresume] from [B2]
)

select [clientID],SUM([Inresume]) as leftmoney
 from t 
group by[clientID]

/*
clientID    leftmoney
a    2000
b    2000
c    1000
*/