select * from 表A a,表B b where a.id=b.infoid
and not exists(select 1 from 表B where infoid=b.infoid and comid>a.comid)

------解决方案--------------------
SQL code
select * from 表A a,表B b where a.id=b.infoid
and not exists(select 1 from 表B where infoid=b.infoid and comid>b.comid)

------解决方案--------------------

SQL code
select a.id,a.title,a.date,b.comid,b.detail,b.username
from 表A a left join 表B b on a.id=b.infoid
where not exists(select 1 from 表B where infoid=b.infoid and comid>b.comid)

------解决方案--------------------
SQL code
select
  a.id,a.title,a.date,b.comid,b.detail,b.username
from
  表a a join 表b b
on
  a.id=b.infoid
where
  a.date=(select max(date) from 表a where id=a.id)