SELECT sc.sid,s.sname,max(sc.score) maxscore
FROM `sc_xxx` sc
left join `s_xxx` s on s.sid=sc.sid
group by sid
order by avg(score) limit 1
select a.sname
,a.max_score
from
(
select t1.sname
,avg(t.score) avg_score
,max(t.score) max_score
from sc t
join s t1
on t.sid =t1.sid
group by t1.name
order by avg(t.score )
) a
where rownum= 1;