前1名左连接

Question

鉴于下面的查询,dps_markers中可能有多行具有相同的标记键,但我们只想加入第一行.如果我接受此查询并删除顶部1和ORDER BY我得到一个mbg.marker_value的值但是按原样运行它总是返回null

SELECT u.id, mbg.marker_value 
FROM dps_user u
LEFT JOIN 
    (SELECT TOP 1 m.marker_value, um.profile_id
     FROM dps_usr_markers um (NOLOCK)
         INNER JOIN dps_markers m (NOLOCK) 
             ON m.marker_id= um.marker_id AND 
                m.marker_key = 'moneyBackGuaranteeLength'
     ORDER BY m.creation_date
    ) MBG ON MBG.profile_id=u.id 
WHERE u.id = 'u162231993'

Answer 1

使用OUTER APPLY而不是LEFT JOIN:

SELECT u.id, mbg.marker_value 
FROM dps_user u
OUTER APPLY 
    (SELECT TOP 1 m.marker_value, um.profile_id
     FROM dps_usr_markers um (NOLOCK)
         INNER JOIN dps_markers m (NOLOCK) 
             ON m.marker_id= um.marker_id AND 
                m.marker_key = 'moneyBackGuaranteeLength'
     WHERE um.profile_id=u.id 
     ORDER BY m.creation_date
    ) AS MBG
WHERE u.id = 'u162231993';

与JOIN不同,APPLY允许您在内部查询中引用u.id.

Answer 2

调试此类情况的关键是单独运行子查询/内联视图以查看输出是什么：

  SELECT TOP 1 
         dm.marker_value, 
         dum.profile_id
    FROM DPS_USR_MARKERS dum (NOLOCK)
    JOIN DPS_MARKERS dm (NOLOCK) ON dm.marker_id= dum.marker_id 
                                AND dm.marker_key = 'moneyBackGuaranteeLength'
ORDER BY dm.creation_date

运行它，您会看到该值与 的值profile_id不匹配，这可以解释为什么任何引用都会返回（感谢左连接；如果它是内部连接，您将不会得到任何东西）。u.idu162231993mbgnull

您已经使用 编写了自己的代码TOP，因为现在如果您想为其他用户运行查询，则必须调整查询。更好的方法是：

   SELECT u.id, 
          x.marker_value 
     FROM DPS_USER u
LEFT JOIN (SELECT dum.profile_id,
                  dm.marker_value,
                  dm.creation_date
             FROM DPS_USR_MARKERS dum (NOLOCK)
             JOIN DPS_MARKERS dm (NOLOCK) ON dm.marker_id= dum.marker_id 
                                         AND dm.marker_key = 'moneyBackGuaranteeLength'
           ) x ON x.profile_id = u.id
     JOIN (SELECT dum.profile_id,
                  MAX(dm.creation_date) 'max_create_date'
             FROM DPS_USR_MARKERS dum (NOLOCK)
             JOIN DPS_MARKERS dm (NOLOCK) ON dm.marker_id= dum.marker_id 
                                         AND dm.marker_key = 'moneyBackGuaranteeLength'
         GROUP BY dum.profile_id) y ON y.profile_id = x.profile_id
                                   AND y.max_create_date = x.creation_date
    WHERE u.id = 'u162231993'

这样，您可以更改子句id中的值where来检查系统中任何用户的记录。