Question

我有一个查询，需要能够在正确的位置连接一个字段，条件是该字段等于变量。

类似的东西：

CASE WHEN challenges.userID = $var
LEFT JOIN challengesRead ON challenges.userID = challengesRead.userID
CASE WHEN challenges.opponentID = $var
LEFT JOIN challengesRead ON challenges.opponentID = challengesRead.userID

我认为我走在正确的轨道上，但我不确定如何将查询放在一起。感谢

Answer 1

这样的事可能有效

LEFT JOIN challengesRead 
ON challenges.userID = CASE 
WHEN challenges.userID = $var THEN challengesRead.userID 
WHEN challenges.opponentID = $var THEN challenges.opponen END

Answer 2

看来你想要这个：

LEFT JOIN challengesRead 
  ON CASE WHEN challenges.userID = $var
            THEN challenges.userID 
          WHEN challenges.opponentID = $var
            THEN challenges.opponentID
     END = challengesRead.userID

可以简化为：

LEFT JOIN challengesRead 
  ON $var = challengesRead.userID
  AND ( challenges.userID = $var
     OR challenges.opponentID = $var
      )

如何使用case语句来确定要连接的字段

2 个答案: