select a.name_drug,a.s_quantity,isnull(b.s_quantity_sale,0) as s_quantity_sale
from
(select name_drug, Sum(quantity) As S_quantity From drug Group By name_drug) a
left outer join (select name_drug, Sum(quantity_sale) As S_quantity_sale From drug_sale WHERE month(date_sale) = 4 Group By name_drug ) b
on a.name_drug = b.name_drug