如何为重复行返回空值？

Question

我在查询中有这个地址：

CASE    
   WHEN  olnn.olnnNote like '%SEE DRAWING%'  
     THEN  'Y:\300 ORDER PROCESSING\Majid Ahmadi\Custom Cabs\' + 'CUSTOM' + '_' + CAST (olnav.olnavValue as nvarchar(25)) +  '.jpg'   --Added 2018-05-23 by MA  
   ELSE Null        
END AS imageFilePath

由于olnn.olnnNote中的重复单元格，导致重复的文件路径。我尝试使用SELECT DISTINCT CASE但我仍然得到了相同的结果。

任何人都知道我的错误？

谢谢

Answer 1

首先，您永远不需要在CASE语句中包含ELSE NULL，因为在没有条件时NULL是默认值。

要获得所需的结果，您可以使用ROW_NUMBER，PARTITION BY您不想复制的行，然后在ROW_NUMBER = 1时返回文件路径。请注意以下示例数据和解决方案：

-- Sample data
DECLARE @olnn TABLE (olnavValue varchar(20), olnnNote varchar(1000));
INSERT @olnn (olnavValue, olnnNote) VALUES 
('abc', 'blah blah SEE DRAWING'),
('abc', 'blah SEE DRAWING blah blah'),
('xxx', 'blah blah blah blah... SEE DRAWING') ,
('yyy', 'SEE DRAWING Please'),
('zzz', 'blah blah blah blah... SEE DRAWING'),
('zzz', 'yada yada... SEE DRAWING'),
('zzz', 'SEE DRAWING');

-- Solution    
WITH olnn AS 
(
  SELECT 
    olnn.olnavValue,
    olnn.olnnNote,
    rn = ROW_NUMBER() OVER (PARTITION BY olnn.olnavValue ORDER BY (SELECT NULL))
  FROM @olnn olnn
)
SELECT 
  olnn.olnavValue,
  FilePath = 
    CASE WHEN olnn.olnnNote LIKE '%SEE DRAWING%' AND olnn.rn = 1 
         THEN 'Y:\300 ORDER PROCESSING\Majid Ahmadi\Custom Cabs\' + 'CUSTOM' + '_' +
           CAST(olnn.olnavValue as nvarchar(25)) +  '.jpg' 
  END
FROM olnn;

<强>结果

olnavValue           FilePath
-------------------- -------------------------------------------------------------------------------------
abc                  Y:\300 ORDER PROCESSING\Majid Ahmadi\Custom Cabs\CUSTOM_abc.jpg
abc                  NULL
xxx                  Y:\300 ORDER PROCESSING\Majid Ahmadi\Custom Cabs\CUSTOM_xxx.jpg
yyy                  Y:\300 ORDER PROCESSING\Majid Ahmadi\Custom Cabs\CUSTOM_yyy.jpg
zzz                  Y:\300 ORDER PROCESSING\Majid Ahmadi\Custom Cabs\CUSTOM_zzz.jpg
zzz                  NULL
zzz                  NULL

请注意，当你没有指定olnnNote不喜欢的时候你会看到什么＆＃39;％SEE DRAWING％＆＃39;。