MySQL INNER连接取决于php变量是否等于字段

I am struggling with an SQL query on the inner join. I want to have it so it joins on the field not the same as the $user_id. Hope that made sense, this might be easier to understand:

if ($user_id = wallfriends.mem_id_from) {
INNER JOIN wallfriends ON wallusers.mem_id = wallfriends.mem_id_to
} else if ($user_id = wallfriends.mem_id_to) {
INNER JOIN wallfriends ON wallusers.mem_id = wallfriends.mem_id_from
}

My Current query:

SELECT * FROM wallusers 
INNER JOIN wallfriends ON wallusers.mem_id = wallfriends.mem_id_from
WHERE ((wallfriends.mem_id_from =".$user_id.") OR (wallfriends.mem_id_to =".$user_id.")) AND wallfriends.confirm=1

Thanks in advance

I think what you need is a putting a ternary comparison on the sql string

$sql = " SELECT * FROM wallusers 
INNER JOIN wallfriends ON wallusers.mem_id = ".( $user_id == "wallfriends.mem_id_to"?"wallfriends.mem_id_from":"wallfriends.mem_id_to")."
WHERE wallfriends.confirm=1";

Try this ::

SELECT * 
FROM wallusers 
INNER JOIN wallfriends ON wallusers.mem_id = wallfriends.mem_id_from
WHERE 
(wallfriends.mem_id_from =".$user_id.") 
OR 
(wallfriends.mem_id_to =".$user_id.")

SELECT * FROM wallusers 
INNER JOIN wallfriends ON 
wallusers.mem_id = 
CASE WHEN 
wallfriends.mem_id_from =".$user_id." 
THEN wallfriends.mem_id_to 
ELSE wallfriends.mem_id_from
END
WHERE wallfriends.confirm=1