--> 测试数据: @a表
declare @a表 table (km int,je int)
insert into @a表
select 1,5 union all
select 2,10 union all
select 3,15 union all
select 4,20

--> 测试数据: @b表
declare @b表 table (bh int,lx int)
insert into @b表
select 1,0 union all
select 2,1 union all
select 3,1 union all
select 4,0 union all
select 5,1

--> 测试数据: @c表
declare @c表 table (bh int,sl varchar(2))
insert into @c表
select 1,'+' union all
select 2,'-' union all
select 3,'+' union all
select 4,'-' union all
select 5,'-'

select c.*,b.lx,
case when c.sl='+' and b.lx=0 then (select je from @a表 where km=1)
when c.sl='-' and b.lx=0 then (select je from @a表 where km=2)
when c.sl='+' and b.lx=1 then (select je from @a表 where km=3)
when c.sl='-' and b.lx=1 then (select je from @a表 where km=4)
end as je
from @c表 c left join @b表 b on c.bh=b.bh

/*
bh          sl   lx          je
----------- ---- ----------- -----------
1           +    0           5
2           -    1           20
3           +    1           15
4           -    0           10
5           -    1           20
*/