SQL面试题挑战13：分组topN

问题：

下面是某个班级的成绩表，需要筛选出每个科目前2名的学生信息。如果分数一样，名次是并列的，后面的同学名次就不连续。比如有2个同学是第一名，那么下一个同学的名次就是第3名，呈现1,1,3的名次排列。

stu_id  stu_name    subject     score
1001    张三        语文           90
1001    张三        数学           80
1001    张三        英语           70
1002    李四        语文           90
1002    李四        数学           75
1002    李四        英语           80
1003    王五        语文           80
1003    王五        数学           70
1003    王五        英语           60

SQL解答：

非常典型的分组topN的问题，是面试经常被问到的。直接使用开窗函数排名即可，注意下row_number、rank、dense_rank三个开窗函数不同场景下的使用。

with temp as
(
    select 1001 as stu_id,'张三' as stu_name,'语文' as subject,90 as score
    union all
    select 1001 as stu_id,'张三' as stu_name,'数学' as subject,80 as score
    union all
    select 1001 as stu_id,'张三' as stu_name,'英语' as subject,70 as score
    union all
    select 1002 as stu_id,'李四' as stu_name,'语文' as subject,90 as score
    union all
    select 1002 as stu_id,'李四' as stu_name,'数学' as subject,75 as score
    union all
    select 1002 as stu_id,'李四' as stu_name,'英语' as subject,80 as score
    union all
    select 1003 as stu_id,'王五' as stu_name,'语文' as subject,80 as score
    union all
    select 1003 as stu_id,'王五' as stu_name,'数学' as subject,70 as score
    union all
    select 1003 as stu_id,'王五' as stu_name,'英语' as subject,60 as score
)
select
stu_id
,stu_name
,subject
,score
from
(
   select 
    stu_id
    ,stu_name
    ,subject
    ,score
    ,rank() over(partition by subject order by score desc) as rk
    from temp
) t1
where rk<=2;

---结果：
stu_id		stu_name		subject			score	
1001          张三            数学           80	
1002          李四            数学           75
1002          李四            英语           80	
1001          张三            英语           70	
1001          张三            语文           90
1002          李四            语文           90