相加query的求教！

Question

select
    a.Name,
    a.cost + b.cost
from
    (select 
         Assignment.Name, 
         sum(People.Cost) as cost
     from 
         Assignment, 
         People
     where 
         Assignment.AssID=People.AssID
     Group by 
         Assignment.Name) a,
    (select 
         Assignment.Name, 
         sum(Equipment.Cost) as cost
     from 
         Assignment, 
         Equipment
     where 
         Assignment.AssID=Equipment.AssID
     Group by 
         Assignment.Name) b
where
    a.Name = b.Name