mysql

#查询score中最高分的学生学号
SELECT sno,cno from score ORDER BY degree DESC LIMIT 0,1;
#查询每一们课程的平均成绩
 SELECT cno,avg(degree) from score GROUP BY cno;
#查询至少5门，并且是以3开头的
select avg(Degree) from score where Cno like '3%' and group by Cno having count(*)>=5

链接要小表驱动大表，
所以如果是inner join，无所谓
left join 把大表放在后边
right join把大表放在前面

select a.*, b.* from tablea a  
left outer join tableb b  
on a.id = b.id  

select a.id aid,a.age,b.id bid,b.name from tablea a  
left join tableb b  
on a.id = b.id  
Where b.id is null

select a.id aid,a.age,b.id bid,b.name from tablea a  
left join tableb b  
on a.id = b.id  
union  
select a.id aid,a.age,b.id bid,b.name from tablea a  
right join tableb b  
on a.id = b.id  


select a.id aid,a.age,b.id bid,b.name from tablea a  
left join tableb b  
on a.id = b.id  
where b.id is null  
union  
select a.id aid,a.age,b.id bid,b.name from tablea a  
right join tableb b  
on a.id = b.id  
where a.id is null