转换单列成

Question

我有每个记录代表一个人的桌子以及用于指示他们参加什么活动的列列基于价值观多行：

CREATE TABLE EventAttendees 
(
    Person VARCHAR(100), 
    [Event A] VARCHAR(1), 
    [Event B] VARCHAR(1), 
    [Event C] VARCHAR(1) 
) 

INSERT INTO EventAttendees 
SELECT 'John Smith','x',NULL,NULL 
UNION 
SELECT 'Jane Doe',NULL,'x','x' 
UNION 
SELECT 'Phil White','x',NULL,'x' 
UNION 
SELECT 'Sarah Jenkins','x','x','x' 

它看起来像这样的例子：

SELECT * FROM Event Attendees 

/---------------|---------|---------|---------\ 
| Person  | Event A | Event B | Event C | 
|---------------|---------|---------|---------| 
| John Smith | x | NULL | NULL | 
| Jane Doe  | NULL | x | x | 
| Phil White | x | NULL | x | 
| Sarah Jenkins | x | x | x | 
\---------------|---------|---------|---------/ 

我要生成谁出席了活动，所以我期望的输出是一个列表：

/---------------|---------| 
| Person  | Event | 
|---------------|---------| 
| John Smith | Event A | 
| Jane Doe  | Event B | 
| Jane Doe  | Event C | 
| Phil White | Event A | 
| Phil White | Event C | 
| Sarah Jenkins | Event A | 
| Sarah Jenkins | Event B | 
| Sarah Jenkins | Event C | 
\---------------|---------/ 

在现实中，我有超过3个事件，但以上是为了便于解释（这是而不是作业问题btw）。由于事件将来可能会发生变化，而且我无法控制传递的数据，所以我真的需要一个动态解决方案，它可以处理任意数量的可能事件列。

我假设我可以用UNPIVOT做一些事情，但我无法弄清楚，或找到一个很好的例子，在SO或其他地方工作 - 有人可以帮忙吗？

Answer 1

试着这么做

SELECT * FROM (
    SELECT Person, CASE WHEN [Event A] = 'x' THEN 'Event A' END AS [Event] FROM EventAttendees 
    UNION 
    SELECT Person, CASE WHEN [Event B] = 'x' THEN 'Event B' END AS [Event] FROM EventAttendees 
    UNION 
    SELECT Person, CASE WHEN [Event C] = 'x' THEN 'Event C' END AS [Event] FROM EventAttendees 
    ) AS EventAttendees 
    WHERE Event is not null 
    order by Person 

动态SQL你可以尝试这样的事：

DECLARE @name varchar(30) 
    DECLARE @sql varchar(1000) = 'SELECT * FROM ('; 
    DECLARE NameCursor CURSOR 
     FOR select name from sys.all_columns where object_id = (select object_id from sys.tables where name='EventAttendees') and name!='Person' 
    OPEN NameCursor 
    FETCH NEXT FROM NameCursor INTO @name 

    WHILE @@FETCH_STATUS = 0 
    BEGIN 

     SET @sql += 'SELECT Person, CASE WHEN [' + @name+'] = ''x'' THEN ''' + @name +''' END AS [Event] FROM EventAttendees' 
     FETCH NEXT FROM NameCursor INTO @name 

     IF(@@FETCH_STATUS = 0) 
     BEGIN 
      SET @sql += ' UNION '; 
     END 

    END; 
    CLOSE NameCursor; 
    DEALLOCATE NameCursor; 
    SET @sql += ') AS EventAttendees 
      WHERE Event is not null 
      order by Person'; 

    execute (@sql); 

Answer 2

我做到这一点使用outer apply：

select ea.person, v.EventName 
from EventAttendees ea outer apply 
    (values ('Event A', [Event A]), 
      ('Event B', [Event B]), 
      ('Event C', [Event C]) 
    ) v(EventName, EventFlag) 
where v.EventFlag = 'x' 

Answer 3

你可以用逆透视做到这一点正如你所说的，你只需要确保你告诉它是什么事件，否则你只会得到一个X：

CREATE TABLE #tmpEventAttendees 
(
    Person VARCHAR(100), 
    [Event A] VARCHAR(1), 
    [Event B] VARCHAR(1), 
    [Event C] VARCHAR(1) 
) 
INSERT INTO #tmpEventAttendees 
SELECT 'John Smith','x',NULL,NULL 
UNION 
SELECT 'Jane Doe',NULL,'x','x' 
UNION 
SELECT 'Phil White','x',NULL,'x' 
UNION 
SELECT 'Sarah Jenkins','x','x','x' 

SELECT Person, [Event] 
FROM 
(
    SELECT Person                     , 
      CASE WHEN [Event A] IS NOT NULL THEN 'Event A' END AS [Event A]        , 
      CASE WHEN [Event B] IS NOT NULL THEN 'Event B' END AS [Event B]        , 
      CASE WHEN [Event C] IS NOT NULL THEN 'Event C' END AS [Event C] 
    FROM #tmpEventAttendees 
) AS cp 
UNPIVOT 
(
    [Event] FOR [Events] IN ([Event A], [Event B], [Event C]) 
) AS up; 

DROP TABLE #tmpEventAttendees 

Answer 4

想通了，我想的解决方案，但肯定的，它确实需要动态SQL来获得相关列名送入UNPIVOT：

declare @sql varchar(max) 
set @sql = 
    'select Person, EventName 
    from EventAttendees 
    unpivot 
    (
     Attended for EventName in (' + (select 
             stuff((
              select ',' + QUOTENAME(c.[name]) 
              from sys.columns c 
              join sys.objects o on c.object_id = o.object_id 
              where o.[name] = 'EventAttendees' 
              and c.column_id > 1 
              order by c.[name] 
              for xml path('') 
             ),1,1,'') as colList) + ') 
    ) unpiv 
    where unpiv.Attended = ''x'' 
    order by Person, EventName' 

exec (@sql) 

在这个例子中，我提出假设Event列是从表中的第二列开始的，但显然我可以在子查询中使用一些不同的逻辑来确定相关列（如有必要）。

在我的示例数据，这给期望的结果：

/---------------------------\ 
| Person  | EventName | 
|---------------|-----------| 
| Jane Doe  | Event B | 
| Jane Doe  | Event C | 
| John Smith | Event A | 
| Phil White | Event A | 
| Phil White | Event C | 
| Sarah Jenkins | Event A | 
| Sarah Jenkins | Event B | 
| Sarah Jenkins | Event C | 
\---------------------------/ 

我想我更喜欢这对使用游标，虽然我还没有真正证实了性能上的差异（如果有的话）存在之间两种动态方法。

感谢大家对这个问题的帮助和建议，尽管如此，非常感谢！