SQL分组后取topN
准备数据
create table SC(
SId varchar(10) comment "学生ID",
CId varchar(10) comment "课程ID",
score decimal(18,1) comment "课程成绩");
insert into SC values('01' , '01' , 80);
insert into SC values('01' , '02' , 90);
insert into SC values('01' , '03' , 99);
insert into SC values('02' , '01' , 70);
insert into SC values('02' , '02' , 60);
insert into SC values('02' , '03' , 80);
insert into SC values('03' , '01' , 80);
insert into SC values('03' , '02' , 80);
insert into SC values('03' , '03' , 80);
insert into SC values('04' , '01' , 50);
insert into SC values('04' , '02' , 30);
insert into SC values('04' , '03' , 20);
insert into SC values('05' , '01' , 76);
insert into SC values('05' , '02' , 87);
insert into SC values('06' , '01' , 31);
insert into SC values('06' , '03' , 34);
insert into SC values('07' , '02' , 89);
insert into SC values('07' , '03' , 98);1、TOP 1
求每门课程中成绩最好的人
使用窗口函数:
SELECT SId ,CId,score from (
SELECT SId,CId,score,dense_rank() over (partition by CId ORDER BY score desc) as ranking from sc ) new
where ranking=1利用表链接
SELECT a.*,b.max_score from sc a
INNER JOIN (
SELECT CId,max(score) max_score
from sc
GROUP BY CId ) b
on a.CId= b.CId and a.score= b.max_score;使用子查询
SELECT * from sc a
where score = (SELECT max(score) from sc where a.CId=CId )2、TOP N
求每门课程成绩前两人
使用窗口函数:
SELECT SId,CId,score from (
SELECT SId,CId,score,rank() over (PARTITION by CId ORDER BY score DESC) as ranking
from sc ) b
where ranking<=2;使用自身左链接
SELECT SId,CId,score from (
SELECT a.*
from sc a left JOIN sc b
on a.CId= b.CId and a.score 结果：
3、三种排名的方式
#这是代码
SELECT SId,CId,score,
row_number() over (partition by CId ORDER BY score desc) as row_number2,
rank() over (partition by CId ORDER BY score desc) as rank2,
dense_rank() over (partition by CId ORDER BY score desc) as dense_rank2
from sc
where CId = "01"；这是对CID分组，按分数排名的表
上图中3中不同的排名方式，对应sql窗口函数的三种
row_number : 就是按1-2-3顺序排
rank:就是按 1-1-3 排列--这就是 美国式排名
dense_rank:就是按 1-1-2-2-3排列--这就是中国式排名
请牢记这三种排名哦！！！
应用：
需求：求每个班级每科成绩的第一名的学生
select name,subject,score,class
from (select name,subject,score,class,rank()
over (partition by class,subject order by score desc) as rank from score) score_tmp
where rank = 1;