If your hierarchy is only one level deep as in the sample data:

WITH(CTE_TableA) AS

(

SELECT JoinKey = COALESCE(ParentID,ID)

FROM tabA

)

SELECT ID = A.JoinKey, Value = SUM(B.Value)

FROM

CTE_TableA A

INNER JOIN

tabB B

ON A.JoinKey = B.ID

GROUP BY A.JoinKey

If you have more levels in your hierarchy, you can use a recursive CTE to flatten the hierarchy and then do the aggregation.