Which days had a minimum dew point smaller than any day in zip code 94107, and in which zip codes were those measurements taken?
SELECT date ,  zip_code FROM weather WHERE min_dew_point_f  <  (SELECT min(min_dew_point_f) FROM weather WHERE zip_code  =  94107)
minimum dew point smaller than any day in zip code 94107 refers to min_dew_point_f < MIN(min_dew_point_f) WHERE zip_code = 94107; which days and in which zip codes were those measurements taken refers to SELECT date, zip_code;