delimiter //
CREATE TRIGGER trig_game_uniqueness_check BEFORE INSERT ON chess_games
FOR EACH ROW
  BEGIN
    SELECT `id` INTO TMP_ID FROM `chess_games`
        WHERE `whiteUserID` = [HOW DO I ACCESS THIS?]
            AND `blackUserID` = [HOW DO I ACCESS THIS?]
            AND `outcome` = -1
            AND `tournamentID` IS NULL;
    IF TMP_ID IS NULL
        [HOW DO I ABORT INSERTION?]
    END IF;
  END
delimiter ;