Calculate the total home team goal scored by players whose age are 30 years old and below.
age are 30 years old and below refers to SUBTRACT(datetime(CURRENT_TIMESTAMP,'localtime'), datetime(birthday) < 31)
SELECT SUM(t2.home_team_goal) FROM Player AS t1 INNER JOIN match AS t2 ON t1.player_api_id = t2.away_player_1 WHERE datetime(CURRENT_TIMESTAMP, 'localtime') - datetime(T1.birthday) < 31