What is the first name of the aldermen of wards with more than 50,000 inhabitants?
more than 50000 inhabitants refers to Population > 50000; first name of alderman refers to alderman_first_name
SELECT alderman_first_name FROM Ward WHERE Population > 50000
database name:db_id chicago_crime
Community_Area table Community_Area Cols: community area number dtype integer, community area name dtype text, side dtype text, population dtype text, 
District table District Cols: district number dtype integer, district name dtype text, address dtype text, zip code dtype integer, commander dtype text, email dtype text, phone dtype text, fax dtype text, tty dtype text, twitter dtype text, 
FBI_Code table FBI_Code Cols: fbi code number dtype text, title dtype text, description dtype text, crime against dtype text, 
IUCR table IUCR Cols: iucr number dtype text, primary description dtype text, secondary description dtype text, index code dtype text, 
Neighborhood table Neighborhood Cols: neighborhood name dtype text, community area number dtype integer, 
Ward table Ward Cols: ward_no dtype integer, alderman_first_name dtype text, alderman_last_name dtype text, alderman_name_suffix dtype text, ward_office_address dtype text, ward_office_zip dtype text, ward_email dtype text, ward_office_phone dtype text, ward_office_fax dtype text, city_hall_office_room dtype integer, city_hall_office_phone dtype text, city_hall_office_fax dtype text, Population dtype integer, 
Crime table Crime Cols: report number dtype integer, case number dtype text, date dtype text, block dtype text, Illinois Uniform Crime Reporting number dtype text, location description dtype text, arrest dtype text, domestic dtype text, beat dtype integer, district number dtype integer, ward number dtype integer, community area number dtype integer, fbi code number dtype text, latitude dtype text, longitude dtype text, 
