using distinct in Mysql query


I am trying to build a high score list of 5 entries per game level,
but with only at most 1 entry per player per level. The following did
not work:

    @levels = Level.find(:all, :select => 'distinct user_id', :include
=> [:user], :conditions => ["level_number = ?",
params[:level]], :order => "moves, time", :limit => 5)

User_id is the column that identifies each player in the db. A player
can have many entries per level (all games are saved), but I only want
to show each person once on the list.