Вопрос по запросу

declare @tbl1 table( x int)
insert into @tbl1 values (1),(2)

declare @tbl2 table (a int,b int)
insert into @tbl2 values 
(1,1),
(2,2),
(4,2),
(7,2)

select sum( case when b in (select x from @tbl1) then a else -a end)
from @tbl2
group by b