What is the number of days that had an average humity above 50 and an average visibility above 8?
return days ;return humity of #1 ;return average  of #2 for each #1 ;return #1 where #3 is  higher than 50 ;return visibility of #1 ;return average  of #5 ;return #1 where #6 is  higher than 8 ;return #1 of both #4 and #7 ;return number of  #8
['select', 'project', 'group', 'comparative', 'project', 'aggregate', 'comparative', 'intersection', 'aggregate']