尝试使用CASE的结果作为Inner Join的标准，我做错了什么？

I'm working on a project where I need to do a table join with one of two columns depending on a given row's contents. Basically, given a specific Username, I'd like to obtain information about all of their opponents in all the games they're playing. The two tables I'm using here are GameSession and Users. GameSession has a column for each of the players(Player1 and Player2), which are foreign keys in the Users table column Username, which also contains a Rank column. Here's my select statment:I'm working on a project where I need to do a t