Find the average age of all students living in the each city.
SELECT avg(age) ,  city_code FROM student GROUP BY city_code
SELECT avg(age) ,  city_code FROM student GROUP BY city_code
[{'avg(age)': 20.0, 'city_code': 'ATL'}, {'avg(age)': 18.5, 'city_code': 'BAL'}, {'avg(age)': 18.0, 'city_code': 'BOS'}, {'avg(age)': 20.0, 'city_code': 'CHI'}, {'avg(age)': 18.0, 'city_code': 'DAL'}, {'avg(age)': 17.0, 'city_code': 'DET'}, {'avg(age)': 18.0, 'city_code': 'HKG'}, {'avg(age)': 17.0, 'city_code': 'HOU'}, {'avg(age)': 27.0, 'city_code': 'LON'}, {'avg(age)': 18.0, 'city_code': 'LOS'}, {'avg(age)': 18.0, 'city_code': 'NAR'}, {'avg(age)': 20.333333333333332, 'city_code': 'NYC'}, {'avg(age)': 17.0, 'city_code': 'PEK'}, {'avg(age)': 19.666666666666668, 'city_code': 'PHL'}, {'avg(age)': 19.0, 'city_code': 'PIT'}, {'avg(age)': 20.0, 'city_code': 'ROC'}, {'avg(age)': 21.0, 'city_code': 'SFO'}, {'avg(age)': 21.666666666666668, 'city_code': 'WAS'}, {'avg(age)': 22.5, 'city_code': 'YYZ'}]