### Instruction:

Convert text to sql: What is the average number of attendance at home games for each year? | all_star : player_id , year , game_num , game_id , team_id , league_id , gp , starting_pos | appearances : year , team_id , league_id , player_id , g_all , gs , g_batting , g_defense , g_p , g_c , g_1b , g_2b , g_3b , g_ss , g_lf , g_cf , g_rf , g_of , g_dh , g_ph , g_pr | manager_award : player_id , award_id , year , league_id , tie , notes | player_award : player_id , award_id , year , league_id , tie , notes | manager_award_vote : award_id , year , league_id , player_id , points_won , points_max , votes_first | player_award_vote : award_id , year , league_id , player_id , points_won , points_max , votes_first | batting : player_id , year , stint , team_id , league_id , g , ab , r , h , double , triple , hr , rbi , sb , cs , bb , so , ibb , hbp , sh , sf , g_idp | batting_postseason : year , round , player_id , team_id , league_id , g , ab , r , h , double , triple , hr , rbi , sb , cs , bb , so , ibb , hbp , sh , sf , g_idp | player_college : player_id , college_id , year | fielding : player_id , year , stint , team_id , league_id , pos , g , gs , inn_outs , po , a , e , dp , pb , wp , sb , cs , zr | fielding_outfield : player_id , year , stint , glf , gcf , grf | fielding_postseason : player_id , year , team_id , league_id , round , pos , g , gs , inn_outs , po , a , e , dp , tp , pb , sb , cs | hall_of_fame : player_id , yearid , votedby , ballots , needed , votes , inducted , category , needed_note | home_game : year , league_id , team_id , park_id , span_first , span_last , games , openings , attendance | manager : player_id , year , team_id , league_id , inseason , g , w , l , rank , plyr_mgr | manager_half : player_id , year , team_id , league_id , inseason , half , g , w , l , rank | player : player_id , birth_year , birth_month , birth_day , birth_country , birth_state , birth_city , death_year , death_month , death_day , death_country , death_state , death_city , name_first , name_last , name_given , weight , height , bats , throws , debut , final_game , retro_id , bbref_id | park : park_id , park_name , park_alias , city , state , country | pitching : player_id , year , stint , team_id , league_id , w , l , g , gs , cg , sho , sv , ipouts , h , er , hr , bb , so , baopp , era , ibb , wp , hbp , bk , bfp , gf , r , sh , sf , g_idp | pitching_postseason : player_id , year , round , team_id , league_id , w , l , g , gs , cg , sho , sv , ipouts , h , er , hr , bb , so , baopp , era , ibb , wp , hbp , bk , bfp , gf , r , sh , sf , g_idp | salary : year , team_id , league_id , player_id , salary | college : college_id , name_full , city , state , country | postseason : year , round , team_id_winner , league_id_winner , team_id_loser , league_id_loser , wins , losses , ties | team : year , league_id , team_id , franchise_id , div_id , rank , g , ghome , w , l , div_win , wc_win , lg_win , ws_win , r , ab , h , double , triple , hr , bb , so , sb , cs , hbp , sf , ra , er , era , cg , sho , sv , ipouts , ha , hra , bba , soa , e , dp , fp , name , park , attendance , bpf , ppf , team_id_br , team_id_lahman45 , team_id_retro | team_franchise : franchise_id , franchise_name , active , na_assoc | team_half : year , league_id , team_id , half , div_id , div_win , rank , g , w , l | all_star.player_id = player.player_id | appearances.player_id = player.player_id | appearances.team_id = team.team_id | manager_award.player_id = player.player_id | player_award.player_id = player.player_id | player_award_vote.player_id = player.player_id | batting.player_id = player.player_id | batting_postseason.team_id = team.team_id | batting_postseason.player_id = player.player_id | player_college.college_id = college.college_id | player_college.player_id = player.player_id | fielding.player_id = player.player_id | fielding_outfield.player_id = player.player_id | fielding_postseason.player_id = player.player_id | hall_of_fame.player_id = player.player_id | home_game.park_id = park.park_id | home_game.team_id = team.team_id | manager.team_id = team.team_id | manager_half.team_id = team.team_id | 

### Response:

select year , avg ( attendance ) from home_game group by year