SQL Server：如何將列轉換爲一行？

Question

我有一列根據結果具有不同的值。 我想返回固定列的一行。

列輸出如下：

Group | Item 
-------------- 
    G1 | 1 
    G1 | 2 
    G2 | 3 

我想輸出：

Group | Item1 | Item2 | Item3 
----------------------------- 
    G1 | 1 | 2 | N/A 
    G2 | 3 | N/A | N/A 

請幫助..提前

謝謝...

Answer 1

因爲每一個組可以有三個項目最大的，我會用PIVOT operator：

DECLARE @Test TABLE 
(
    RowID INT IDENTITY(1,1) PRIMARY KEY 
    ,[Group]VARCHAR(10) NOT NULL 
    ,Item INT NOT NULL 
    ,UNIQUE ([Group], Item) 
); 
INSERT @Test VALUES ('G1', 1); 
INSERT @Test VALUES ('G1', 2); 
INSERT @Test VALUES ('G2', 3); 

WITH PivotSource 
AS 
(
    SELECT t.[Group], t.Item 
      ,ROW_NUMBER() OVER(PARTITION BY t.[Group] ORDER BY t.RowID) RowNumber 
    FROM @Test t 

) 
SELECT pvt.[Group] 
     ,Item1 = ISNULL(CONVERT(VARCHAR(11), pvt.[1]) , 'N/A') 
     ,Item2 = ISNULL(CONVERT(VARCHAR(11), pvt.[2]) , 'N/A') 
     ,Item3 = ISNULL(CONVERT(VARCHAR(11), pvt.[3]) , 'N/A') 
FROM PivotSource src 
PIVOT (MAX(src.Item) FOR src.RowNumber IN ([1], [2], [3])) pvt; 

結果：

​​

Answer 2

由於你有固定數量的輸出列，你可以用ROW_NUMBER()和做一些技巧。

select child1.[Group], child1.Item as Item1, child2.Item as Item2, child3.Item as Item3 from 
(select [Group], Item from 
    (select [Group], Item, (ROW_NUMBER() over(partition by [group] order by item)) as rownum from GroupTable) as child1_inner 
    where child1_inner.rownum = 1) as child1 
left outer join 
(select [Group], Item from 
    (select [Group], Item, (ROW_NUMBER() over(partition by [group] order by item)) as rownum from GroupTable) as child2_inner 
    where child2_inner.rownum = 2) as child2 
on child1.[Group] = child2.[Group] 
left outer join 
(select [Group], Item from 
    (select [Group], Item, (ROW_NUMBER() over(partition by [group] order by item)) as rownum from GroupTable) as child3_inner 
    where child3_inner.rownum = 3) as child3 
on child1.[Group] = child3.[Group] 

這將返回null代替N/A，但您可以修復與COALESCE。

Answer 3

我想這樣的事情會工作：

SELECT t1.[Group], t1.Item, t2.Item, t3.Item 
from tbl t1, tbl t2, tbl t3 
where t1.[Group] = t2.[Group] and t1.[Group] = t3.[Group] 
    and t1.Item < t2.Item and t2.Item < t3.Item 
    and t1.[Group] in (select [Group] from tbl group by [Group] having COUNT(*) = 3) 
union 
SELECT t1.[Group], t1.Item, t2.Item, NULL 
from tbl t1, tbl t2 
where t1.[Group] = t2.[Group] 
    and t1.Item < t2.Item 
    and t1.[Group] in 
    (select [Group] from tbl group by [Group] having COUNT(*) = 2) 
union 
select tbl.[Group], tbl.Item, NULL, NULL 
from tbl 
where [Group] in (select [Group] from tbl group by [Group] having COUNT(*) = 1)