Question

我正在尝试这样做......

SELECT lc.Location, count(h.empID) as 'Count', lu.LBL
FROM swam.lookup lu
    LEFT JOIN swam.empTable h
    ON CASE
        WHEN lu.sac is null 
            THEN lu.POS = h.POS
        ELSE (
            lu.POS = h.POS
            and
            lu.sac = h.sac)
    INNER JOIN swam.Locations lc
        ON h.LocationID = lc.LocationID
GROUP BY lc.Location, lu.LBL
ORDER BY lc.Location

因此，如果lu.sac为null，则只将表连接到一个公共列上。如果它不为空，那么它必须同时使用POS和SAC加入。

这可能吗？

Answer 1

不要使用case。只需使用直接布尔逻辑：

FROM swam.lookup lu LEFT JOIN
     swam.empTable h
     ON lu.POS = h.POS AND
        (lu.sac is null OR lu.sac = h.sac) INNER JOIN
     swam.Locations fs
     ON h.LocationID = lc.LocationID

作为奖励，数据库引擎还可以在任一（或两个）表中使用pos上的索引的优势。

Answer 2

    SELECT lc.Location, count(h.empID) as 'Count', lu.LBL
FROM swam.lookup lu
    LEFT JOIN swam.empTable h
    ON  (lu.sac is null  AND lu.POS = h.POS) OR  
                   not(lu.sac is null)  AND lu.POS = h.POS   and lu.sac = h.sac
    INNER JOIN swam.Locations fs
        ON h.LocationID = lc.LocationID
GROUP BY lc.Location, lu.LBL
ORDER BY lc.Location

Answer 3

SELECT lc.Location, count(h.empID) as 'Count', lu.LBL
FROM swam.lookup lu
LEFT JOIN swam.empTable h
  ON lu.POS = h.POS
 AND isnull(lu.sac, h.sac) = h.sac    
INNER JOIN swam.Locations fs
   ON h.LocationID = lc.LocationID  // this looks wrong lc is not defined
GROUP BY lc.Location, lu.LBL
ORDER BY lc.Location

我认为最后一次加入会使左连接前奏和内连接

加入的“ON”caluse中的大小写

3 个答案: