Mysql 如何使用case语句来确定左连接哪个字段

我有一个查询，在给定字段等于变量的条件下，需要能够在正确的位置左键联接字段

比如：

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

我认为我的思路是对的，但我不确定如何把这个问题组合起来。

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谢谢

像这样的东西可能有用

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

---

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

可简化为：

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