说完Pivot 今天说下Unpivot 的处理方式

上次说到，既然有Pivot 的行转列，那么肯定也有Unpivot 的列转行 。其实unpivot 处理的情况也是差不多，也是分3步走。

首先也是先演示一下unpivot 的用法

CREATE TABLE #Emp (Name NVARCHAR(50),Mon TIME,Tue TIME,Wed TIME,Thu TIME)

INSERT INTO #Emp
        ( Name, Mon, Tue, Wed, Thu )
VALUES  ( N'Ken', '8:30',  '8:45','9:05','8:55' ),
        ( N'Joan', '8:25',  '8:35', NULL,'9:00' ),
        ( N'Jack', '8:15',  '9:03','9:05','8:38' ),
        ( N'Bob', '9:30',  '8:53','9:01','8:56' )    
        
SELECT * FROM #Emp    

Name                                               Mon              Tue              Wed              Thu
-------------------------------------------------- ---------------- ---------------- ---------------- ----------------
Ken                                                08:30:00.0000000 08:45:00.0000000 09:05:00.0000000 08:55:00.0000000
Joan                                               08:25:00.0000000 08:35:00.0000000 NULL             09:00:00.0000000
Jack                                               08:15:00.0000000 09:03:00.0000000 09:05:00.0000000 08:38:00.0000000
Bob                                                09:30:00.0000000 08:53:00.0000000 09:01:00.0000000 08:56:00.0000000

SELECT *
    FROM #Emp UNPIVOT( [LoginTime] FOR [WeekDay] IN (Mon,Tue,Wed,Thu)) a


Name                                               LoginTime        WeekDay
-------------------------------------------------- ---------------- --------------------------------------------------------------------------------------------------------------------------------
Ken                                                08:30:00.0000000 Mon
Ken                                                08:45:00.0000000 Tue
Ken                                                09:05:00.0000000 Wed
Ken                                                08:55:00.0000000 Thu
Joan                                               08:25:00.0000000 Mon
Joan                                               08:35:00.0000000 Tue
Joan                                               09:00:00.0000000 Thu
Jack                                               08:15:00.0000000 Mon
Jack                                               09:03:00.0000000 Tue
Jack                                               09:05:00.0000000 Wed
Jack                                               08:38:00.0000000 Thu
Bob                                                09:30:00.0000000 Mon
Bob                                                08:53:00.0000000 Tue
Bob                                                09:01:00.0000000 Wed
Bob                                                08:56:00.0000000 Thu

解释一下，

UNPIVOT( [LoginTime] FOR [WeekDay] IN (Mon,Tue,Wed,Thu)) 里面 。 WeekDay 是 根据 (Mon,Tue,Wed,Thu) 这4个列聚合起来的新列的名称，而 LoginTime 呢，就是存放 (Mon,Tue,Wed,Thu) 这几个列下面的值的新列名称。用起来还是挺方便的。

然后下面我要讲一下unpivot 的3步新路里程

1、赋值元素。首先是会按照 (Mon,Tue,Wed,Thu )  4个列进行一个行赋值。生成第一步的虚拟表。就像下面一样

SELECT *
    FROM #Emp a
        FULL JOIN (SELECT 'Mon' AS [WeekDay]
                    UNION ALL
                    SELECT 'Tue' AS [WeekDay]
                    UNION ALL
                    SELECT 'Wed' AS [WeekDay]
                    UNION ALL
                    SELECT 'Thu' AS [WeekDay])b ON 1 = 1    

得出中间表
Name                                               Mon              Tue              Wed              Thu              WeekDay
-------------------------------------------------- ---------------- ---------------- ---------------- ---------------- -------
Bob                                                09:30:00.0000000 08:53:00.0000000 09:01:00.0000000 08:56:00.0000000 Mon
Bob                                                09:30:00.0000000 08:53:00.0000000 09:01:00.0000000 08:56:00.0000000 Tue
Bob                                                09:30:00.0000000 08:53:00.0000000 09:01:00.0000000 08:56:00.0000000 Wed
Bob                                                09:30:00.0000000 08:53:00.0000000 09:01:00.0000000 08:56:00.0000000 Thu
Jack                                               08:15:00.0000000 09:03:00.0000000 09:05:00.0000000 08:38:00.0000000 Thu
Jack                                               08:15:00.0000000 09:03:00.0000000 09:05:00.0000000 08:38:00.0000000 Wed
Jack                                               08:15:00.0000000 09:03:00.0000000 09:05:00.0000000 08:38:00.0000000 Tue
Jack                                               08:15:00.0000000 09:03:00.0000000 09:05:00.0000000 08:38:00.0000000 Mon
Joan                                               08:25:00.0000000 08:35:00.0000000 NULL             09:00:00.0000000 Tue
Joan                                               08:25:00.0000000 08:35:00.0000000 NULL             09:00:00.0000000 Mon
Joan                                               08:25:00.0000000 08:35:00.0000000 NULL             09:00:00.0000000 Wed
Joan                                               08:25:00.0000000 08:35:00.0000000 NULL             09:00:00.0000000 Thu
Ken                                                08:30:00.0000000 08:45:00.0000000 09:05:00.0000000 08:55:00.0000000 Thu
Ken                                                08:30:00.0000000 08:45:00.0000000 09:05:00.0000000 08:55:00.0000000 Mon
Ken                                                08:30:00.0000000 08:45:00.0000000 09:05:00.0000000 08:55:00.0000000 Tue
Ken                                                08:30:00.0000000 08:45:00.0000000 09:05:00.0000000 08:55:00.0000000 Wed

2 提取有效元素。结果类似下面语句生成的结果

;WITH CTE AS(
SELECT *
    FROM #Emp a
        FULL JOIN (SELECT 'Mon' AS [WeekDay]
                    UNION ALL
                    SELECT 'Tue' AS [WeekDay]
                    UNION ALL
                    SELECT 'Wed' AS [WeekDay]
                    UNION ALL
                    SELECT 'Thu' AS [WeekDay])b ON 1 = 1)
SELECT Name,[WeekDay],
        CASE [WeekDay] WHEN 'Mon' THEN Mon 
                        WHEN 'Tue' THEN Tue 
                        WHEN 'Wed' THEN Wed 
                        WHEN 'Thu' THEN Thu ELSE NULL END AS LoginTime
    FROM CTE 

Name                                               WeekDay LoginTime
-------------------------------------------------- ------- ----------------
Ken                                                Mon     08:30:00.0000000
Joan                                               Mon     08:25:00.0000000
Jack                                               Mon     08:15:00.0000000
Bob                                                Mon     09:30:00.0000000
Ken                                                Tue     08:45:00.0000000
Joan                                               Tue     08:35:00.0000000
Jack                                               Tue     09:03:00.0000000
Bob                                                Tue     08:53:00.0000000
Ken                                                Wed     09:05:00.0000000
Joan                                               Wed     NULL
Jack                                               Wed     09:05:00.0000000
Bob                                                Wed     09:01:00.0000000
Ken                                                Thu     08:55:00.0000000
Joan                                               Thu     09:00:00.0000000
Jack                                               Thu     08:38:00.0000000
Bob                                                Thu     08:56:00.0000000

3 将Null值元素去除。这个好理解了，中间有Null值的行去除

;WITH CTE AS(
SELECT *
    FROM #Emp a
        FULL JOIN (SELECT 'Mon' AS [WeekDay]
                    UNION ALL
                    SELECT 'Tue' AS [WeekDay]
                    UNION ALL
                    SELECT 'Wed' AS [WeekDay]
                    UNION ALL
                    SELECT 'Thu' AS [WeekDay])b ON 1 = 1),
CTE2 AS (SELECT Name,[WeekDay],
        CASE [WeekDay] WHEN 'Mon' THEN Mon 
                        WHEN 'Tue' THEN Tue 
                        WHEN 'Wed' THEN Wed 
                        WHEN 'Thu' THEN Thu ELSE NULL END AS LoginTime
    FROM CTE )
SELECT *
    FROM CTE2
        WHERE CTE2.LoginTime IS NOT NULL

Name                                               WeekDay LoginTime
-------------------------------------------------- ------- ----------------
Ken                                                Mon     08:30:00.0000000
Joan                                               Mon     08:25:00.0000000
Jack                                               Mon     08:15:00.0000000
Bob                                                Mon     09:30:00.0000000
Ken                                                Tue     08:45:00.0000000
Joan                                               Tue     08:35:00.0000000
Jack                                               Tue     09:03:00.0000000
Bob                                                Tue     08:53:00.0000000
Ken                                                Wed     09:05:00.0000000
Jack                                               Wed     09:05:00.0000000
Bob                                                Wed     09:01:00.0000000
Ken                                                Thu     08:55:00.0000000
Joan                                               Thu     09:00:00.0000000
Jack                                               Thu     08:38:00.0000000
Bob                                                Thu     08:56:00.0000000

  好！然后结果有没有和直接Unpivot 一样呢~揍是一样啦~当然罗~中间语句可能不是我演示的那样，我主要是演示中间的虚拟表~

  好！周四了~再坚持一天~就又可以星期六上班班啦！