Question

给出一个名为“grade”的表：

STUDENT  GRADE
john     94
john     76
john     83
john     87
john     90

我想要一个查询来返回每个学生的几个成绩示例，例如：

STUDENT    GRADE1  GRADE2   GRADE3  GRADE4
John       94      76       83      87

请注意，只返回了4个样本成绩列，但该学生的成绩超过4个。

我只知道如何使用GROUP BY子句中的min（）和max（）函数返回2个示例成绩：

select student, min(grade), max(grade) 
from grades
group by student

是否有除min / max以外的任何技巧或功能，使用GROUP BY子句可以显示超过2个等级？

除非它是查询的一部分而不是存储在数据库中，否则我宁愿不编写自己的存储函数来执行此操作。

除了MIN和MAX之外，我正在考虑从记录集中返回其他值的更多函数（如第二高，第三高等）。

想法？

Answer 1

您可以通过应用row_number()函数然后应用PIVOT：

来获得结果

select student, 
  grade1 = [1], 
  grade2 = [2], 
  grade3 = [3], 
  grade4 = [4]
from
(
  select student, grade,
    row_number() over(partition by student
                      order by grade desc) seq
  from grades
) d
pivot
(
  max(grade)
  for seq in ([1], [2], [3], [4]) -- the # of grades you want returned
) piv;

请参阅SQL Fiddle with Demo。

PIVOT中使用的新列名称将是您想要返回的等级数。分区中使用的ORDER BY是grade desc，但您也可以使用order by newid() to get a random result being returned。

这也可以使用带有CASE表达式的聚合函数来完成：

select student,
  max(case when seq = 1 then grade end) grade1,
  max(case when seq = 2 then grade end) grade2,
  max(case when seq = 3 then grade end) grade3,
  max(case when seq = 4 then grade end) grade4
from
(
  select student, grade,
    row_number() over(partition by student
                      order by newid()) seq
  from grades
) d
group by student;

请参阅SQL Fiddle with Demo

Answer 2

我提出这样的建议。唯一的限制是您无法显示示例1到n，您只能使用确定的数字。

SELECT DISTINCT student, 
(
    select top 1 grade from
    (select grade, ROW_NUMBER() OVER(ORDER BY grade DESC) AS Row from grades g2 where g.student = g2.student) a where a.Row = 1
) AS GRADE1  ,
(
    select top 1 grade from
    (select grade, ROW_NUMBER() OVER(ORDER BY grade DESC) AS Row from grades g2 where g.student = g2.student) a where a.Row = 2
) AS GRADE2,
(
    select top 1 grade from
    (select grade, ROW_NUMBER() OVER(ORDER BY grade DESC) AS Row from grades g2 where g.student = g2.student) a where a.Row = 3
) AS GRADE3,
(
    select top 1 grade from
    (select grade, ROW_NUMBER() OVER(ORDER BY grade DESC) AS Row from grades g2 where g.student = g2.student) a where a.Row = 4
) AS GRADE4
from grades g

使用游标。在此解决方案中，您将获得1到n个结果

DECLARE @grade int
DECLARE @n int

DECLARE @sql varchar(max)

DECLARE _cursor CURSOR FOR 
     SELECT grade
     FROM grade
     WHERE student like 'XXX'

OPEN _cursor
FETCH NEXT FROM _cursor INTO @grade
WHILE @@FETCH_STATUS = 0
BEGIN
    IF @sql like ''
    begin
         set @sql = 'SELECT ' + 'XXX' + ' AS Student '
    end
    set @sql = @sql + ',' + @grade ' as GRADE'+  @n
    set @n = @n + 1
END

CLOSE _cursor
DEALLOCATE _cursor

exec(@sql)

此输出将

STUDENT    GRADE1  GRADE2   GRADE3  GRADE4
John       94      87       83      76

如何让查询返回行值的样本作为列？

2 个答案: