How to use a case statement to determine which field to left join on

Question

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.

Something like:

Answer 1

Something like this may work

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