By how many water boards had the countries water boards decreased by from 1850 to 2011?
return the  country ;return water  boards of #1 ;return #2 in  1850 ;return #2 in  2011 ;return number of  #3 ;return number of  #4 ;return difference of #5 and  #6
['select', 'project', 'filter', 'filter', 'aggregate', 'aggregate', 'arithmetic']