左外连接的最大条件

问题描述

|

----------
samples
----------
SamplesID
stylenumber
stylename
status

-----------
samples_details
-----------
Samples_Details_ID
SamplesID
CustomerName
date_out
date_returned
updated (timestamp)
status

------------
samples_pictures
------------
SamplesPicID
SamplesID

尝试编写一个查询，该查询为我提供了示例的所有行，samples_pictures的所有匹配项（如果有）以及samples_details的最新记录（如果有）。因此，有两个左外部联接，但一个仅检索最新记录。 什么不起作用：

SELECT samples.*,samples_pictures.SamplesPicID,CustomerName,date_out,date_returned,updated,samples_details.status as txn_status 
FROM samples 
    LEFT OUTER JOIN
      ( SELECT Samples_Details_ID,samples_details.status as txn_status,MAX(updated) as MaxUpdated
        FROM samples_details
        GROUP BY Samples_Details_ID
      ) AS MaxTable
      ON MaxTable.SamplesID = samples.SamplesID 
    LEFT OUTER JOIN samples_pictures
      ON samples.SamplesID = samples_pictures.SamplesID

任何帮助将不胜感激！     

解决方法

您的原始查询仍然从samples_details中选择每个记录。 您将必须创建一个子选择，以仅过滤出具有最大时间戳记的那些记录，然后将其与您的samples_details表结合起来以获取其他列。

SELECT   *
FROM     samples s
         LEFT OUTER JOIN sample_details sd ON sd.SamplesID = s.SamplesID
         INNER JOIN (
           SELECT   SamplesID,MAX(update) AS Update
           FROM     samples_details
           GROUP BY
                    SamplesID
         ) sdm ON sdm.SamplesID = sd.SamplesID
         LEFT OUTER JOIN samples_pictures sp ON sp.SamplesID = s.SamplesID

    ,您的select语句中没有

SamplesID

MaxTable

。应该：

SELECT samples.*,samples_pictures.SamplesPicID,CustomerName,date_out,date_returned,updated,samples_details.status as txn_status 
FROM samples 
    LEFT OUTER JOIN (SELECT SamplesID,Samples_Details_ID,samples_details.status as txn_status,MAX(updated) as MaxUpdated FROM samples_details GROUP BY Samples_Details_ID) 
    AS MaxTable ON MaxTable.SamplesID = samples.SamplesID 
LEFT OUTER JOIN samples_pictures ON samples.SamplesID = samples_pictures.SamplesID

    ,

SELECT samples.*,samples_details.status as txn_status
FROM samples 
    LEFT OUTER JOIN samples_details
      ON samples.SamplesID = samples_details.SamplesID
      AND samples_details.updated =
      ( SELECT MAX(updated)
        FROM samples_details
            WHERE SamplesID = samples.SamplesID)
      )
    LEFT OUTER JOIN samples_pictures
      ON samples.SamplesID = samples_pictures.SamplesID

    ,

SELECT samples.*,txn_status 
FROM samples 
    LEFT OUTER JOIN
      ( SELECT Samples_ID,status as txn_status
        FROM samples_details
            INNER JOIN
              ( SELECT Samples_ID,MAX(updated) as MaxUpdated
                FROM samples_details
                GROUP BY Samples_ID,Samples_Details_ID
              ) AS MaxTable
              ON MaxTable.Samples_ID = samples.SamplesID
              AND MaxTable.Samples_Details_ID = samples.Samples_Details_ID
      ) AS MaxJoin
    LEFT OUTER JOIN samples_pictures
      ON samples.SamplesID = samples_pictures.SamplesID