select a.id,a.tid,a.createtime from tb a inner join (select tid,min(createtime) createtime from tb group by tid) b on a.tid=b.tid and a.createtime = b.createtime
select t.* from tabname t where not exists(select 1 from tabname where tid=t.tid and createtime<t.createtime)
(select tid,min(createtime) createtime from tb group by tid) b
on a.tid=b.tid and a.createtime = b.createtime