What are the average, maximum, and minimum number of floors for all buildings?
return buildings ;return floors of  #1 ;return number of  #2 ;return average of  #3 ;return maximum of  #3 ;return minimum of  #3 ;return #4 ,  #5 ,   #6
['select', 'project', 'aggregate', 'aggregate', 'aggregate', 'aggregate', 'union']