如何將查詢返回行值的樣本作爲列？

Question

給定一個名爲表「等級」：

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

我想查詢返回檔次的幾個例子爲每個學生，例如：

STUDENT GRADE1 GRADE2 GRADE3 GRADE4 
John  94  76  83  87 

注意只有4個樣品級列返回，但這個學生有超過4個等級。

我只知道如何返回使用MIN（）和MAX（）函數2個例年級在GROUP BY子句：

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

是否有比最小值/最大值，使其他任何技巧或功能使用GROUP BY子句顯示超過兩個等級？

我寧願不寫我自己的存儲函數來做到這一點，除非它是查詢的一部分，而不是存儲在數據庫中。

我想到更多的功能，從MIN和MAX（如第二高，第三高等），除了記錄集返回其他值。

想法？

Answer 1

我建議這樣的事情。唯一的限制是你不能顯示例子1到n，你僅限於確定的數字。

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 

使用遊標。在這個解決方案，您將有1到N個結果

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) 

這一產量將

STUDENT GRADE1 GRADE2 GRADE3 GRADE4 
John  94  87  83  76 

Answer 2

您可以通過應用row_number()功能，然後應用PIVOT得到的結果：

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; 

見SQL Fiddle with Demo。

在PIVOT中使用的新列名將是您想要返回的等級數。分區中使用的ORDER BY是grade desc，但您也可以使用order by newid() to get a random result being returned來查看。你有一列表示這是第一個，第二個......年級

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; 

見SQL Fiddle with Demo