SELECT f.username, x.reg, ...
    FROM ( SELECT COALESCE(SUM(a.registrations),0) AS reg, ... ) AS a
    JOIN ( SELECT
                  COALESCE ... AS reg,
                  COALESCE ...
                  COALESCE ...
                  COALESCE ...
              FROM finance
              GROUP BY username ) AS x
    JOIN finance AS f USING(username)

SUBSTRING_INDEX(site, "-", 1) = username