如何使用case语句确定要保留连接的字段 [英] How to use a case statement to determine which field to left join on

问题描述

考虑到字段等于变量的条件，我有一个查询需要能够在正确的位置左连接字段.

I have a query that needs to be able to left join a field at the right place given the condition that the field is equal to a variable.

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，但是我不确定如何将查询组合在一起. 谢谢

I think I am on the right track but I'm not sure how to put the query together. Thanks

推荐答案

类似的方法可能有效

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

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