Recursive sum in tree structure

You can use a recursive CTE where you in the anchor part get all rows and in the recursive part join to get the child rows. Remember the original Id aliased RootID from the anchor part and do sum aggregate in the main query grouped by RootID.

SQL Fiddle

MS SQL Server 2012 Schema Setup:

create table T
(
  Id int primary key,
  ParentId int,
  Name varchar(10),
  ProductCount int
);

insert into T values
(1, -1, 'Cars',    0),
(2, -1, 'Bikes',   1),
(3,  1, 'Ford',    10),
(4,  3, 'Mustang', 7),
(5,  3, 'Focus',   4);

create index IX_T_ParentID on T(ParentID) include(ProductCount, Id);

Query 1:

with C as
(
  select T.Id,
         T.ProductCount,
         T.Id as RootID
  from T
  union all
  select T.Id,
         T.ProductCount,
         C.RootID
  from T
    inner join C 
      on T.ParentId = C.Id
)
select T.Id,
       T.ParentId,
       T.Name,
       T.ProductCount,
       S.ProductCountIncludingChildren
from T
  inner join (
             select RootID,
                    sum(ProductCount) as ProductCountIncludingChildren
             from C
             group by RootID
             ) as S
    on T.Id = S.RootID
order by T.Id
option (maxrecursion 0)

Results:

| ID | PARENTID |    NAME | PRODUCTCOUNT | PRODUCTCOUNTINCLUDINGCHILDREN |
|----|----------|---------|--------------|-------------------------------|
|  1 |       -1 |    Cars |            0 |                            21 |
|  2 |       -1 |   Bikes |            1 |                             1 |
|  3 |        1 |    Ford |           10 |                            21 |
|  4 |        3 | Mustang |            7 |                             7 |
|  5 |        3 |   Focus |            4 |                             4 |