There are 2 dimensions in the sales_by_product cube, one is "Products"
and one is "Measures".

After enabling writeback, I update the node ([Mandrake] , [Store
Sales]), which is originally null, using the following MDX statement:

update cube sales_by_product
set ([Mandrake], [Measures].[Store Sales])=1000

(I guarantee Mandrake is a unique member. Anyway, here is microsoft...)

Then I run the following MDX statement to retrieve the affected value:

select
{[Mandrake]} on columns,
{[Measures].Members} on rows
from Sales

The result is still null. Also, retrieving the parent show that the
update is not effective. The case is the same after processing the cube.

However, the row was in fact written to the writeback table. Also, the
updated result is visible in Analysis Manager and Excel (using Pivot
Table). What's the cause of the problem? Any problem in my MDX?

Thanks very much.