How many more volunteers were there in 2015 compared to employees?
return volunteers ;return employees ;return #1 in  2015 ;return #2 in  2015 ;return the  number of  #3 ;return the  number of  #4 ;return the  difference of #5 ,  #6
['select', 'select', 'filter', 'filter', 'aggregate', 'aggregate', 'arithmetic']