左联查询
select * from a left join b on a.id=b.id;
右联查询
select * from a right join b on a.id=b.id;
内联查询
select a.name,b.class from a, b where c.id=b.id;
order by
select *from a order by desc#desc;降序排列，asc升序排列
group by
select count（scores）from group by calss;
having
select count（scores）from a group by class having count(scores>=70) order by desc;
