Why does this work.

How does it know which userid to use on the lw and lm selects? They are foreign keys to the same table, ld_users.

SELECT us.points as userscore,
       us.rank as userrank,
       lw.points as userscoreweekly,
       lw.rank as userrankweekly,
       lm.points as userscoremonthly,
       lm.rank as userrankmonthly
  FROM ld_user_scores as us,
       ld_leaderboard_weekly as lw,
       ld_leaderboard_monthly as lm
 WHERE us.userid = 50

How it should be done:

          SELECT us.points as userscore,
                 us.rank as userrank,
                 lw.points as userscoreweekly,
                 lw.rank as userrankweekly,
                 lm.points as userscoremonthly,
                 lm.rank as userrankmonthly
          FROM ld_user_scores as us
          INNER JOIN ld_leaderboard_weekly as lw ON us.userid = lw.userid
          INNER JOIN ld_leaderboard_monthly as lm ON us.userid = lm.userid
          WHERE us.userid = $userid