select A.*,B.*
from AA as A
inner join BB AS B on A.a_b_code=B.b_code
where A.a_id=(select top 1 a_id from AA where a_b_code=A.a_b_code order by a_time desc,a_id desc)
select A.*,B.*
from AA as A
inner join BB AS B on A.a_b_code=B.b_code
inner join (select a_b_code,max(a_time) as a_time from A group by a_b_code) C
on A.a_b_code=C.a_b_code and A.a_time=C.a_time