Question

我想找到一所学校最小班的最年长的人

我有一张这样的表

School  Class    DOB         Stu_ID 

1       1      2000-10-05     S007  
1       1      2001-08-03     S006
1       2      2000-10-05     S005      
1       2      2001-08-03     S004
2       3      2000-10-05     S003
2       4      2001-08-03     S002
2       4      2000-10-05     S001
2       5      2001-08-03     S009

我需要将结果表格作为

School  Class    DOB         Stu_ID 

1       1      2000-08-03     S006  
2       3      2000-10-05     S003

，即每所学校最小班的最年长的人。

Answer 1

SELECT
    School, Class, DOB, Stu_ID
FROM
    (
    SELECT
        School, Class, DOB, Stu_ID,
        DENSE_RANK() OVER (PARTITION BY School ORDER BY Class, DOB) AS rn
    FROM
        MyTable
    ) X
WHERE X.rn = 1

每个学校重置（PARTITION BY）并从最低等级开始计算，然后从最早的DOB开始计算。

DENSE_RANK允许学生分享生日

Answer 2

您可以使用排名功能：

WITH CTE AS(
    SELECT School, Class, DOB, Stu_ID,
        RN = ROW_NUMBER() OVER (PARTITION BY School ORDER BY Class ASC)
    FROM dbo.Schools
)
SELECT School, Class, DOB, Stu_ID
FROM CTE WHERE RN = 1

Demo

Answer 3

最好的方法是使用窗口函数来计算中间变量：

select School, Class, DOB, Stu_ID 
from (select t.*,
             dense_rank(ClassCnt) over (partition by school) as ClassSizeRank
      from (select t.*,
                   count(*) over (partition by school, class) as ClassCnt,
                   row_number over (partition by school, class order by dob) as AgeSegnum
            from t
           ) t
     ) t
where ageseqnum = 1 and ClassSizeRank = 1;

我不建议在这种类型的查询中使用连接。如果是家庭作业/班级，你应该在问题中指明。

SQL：如何有效地找到具有连接的最小集合中的最小值

3 个答案: