sql两个字段相减语句

方法二

create table 表1( 字段1 varchar(50), 字段2 int)
insert into 表1
select '111001' , 10 union all
select '111002' , 9 union all
select '111003' , 12 union all
select '111004' , 23

create table 表2( 字段1 varchar(50), 字段2 int)
insert into 表2
select '111001' , 3 union all
select '111002' , 2 union all
select '111003' , 12

select a.字段1, (a.[字段2] - (isnull(b.[字段2],0)) ) as 字段2 from 表1 a left join 表2 b
on a.[字段1] = b.[字段1]

结果： 字段1 字段2
111001 7
111002 7
111003 0（不显示）
111004 23

方法三

有两个相同的表t1和t2,都有字段a(字符),b(数值),c(数值),内容如下:
t1 t2
a b c a b c
a1 10 20 a2 2 10
a2 10 20 a3 3 15
a3 10 20
a4 10 20

select t1.a as a,(t1.b - isnull(t2.b,0)) as b,(t1.c - isnull(t2.c,0)) as c from [t1] left join [t2] on (t1.a = t2.a)