分组查询取每组前n条记录实例

假设有这样一张运动员比赛成绩表 tb_score

现在要求查询出每个国家的前三名的成绩记录，查询语句可以这样写：

1、

select t3.id,t3.country,t3.score 
from (select t1.*, (select count(*) from tb_score t2 where t1.score<=t2.score and t1.country=t2.country) as rownum 
			from tb_score t1) t3 
where rownum <=3 order by country,score DESC;

2、

select a.id,a.country,a.name,a.score from tb_score a 
where exists 
(select count(*) from tb_score where country = a.country and score > a.score having Count(*) < 3)
order by a.country,score DESC;

3、

select a.id,a.country,a.name,a.score from tb_score a 
where 
(select count(*) from tb_score where country = a.country and score > a.score )<3
order by a.country,score DESC;

查询结果如下：