I just came across this document in SCN regarding constant selection in Bex , I am sharing here with my detailed screen shots and I am hoping it may be helpful for some of you.
For example, you have a MultiProvider that contains data from the following InfoCubes:
InfoCube 1 :-
InfoCube 2 :-
Since characteristic Customer is contained in InfoCube 2 but not in InfoCube 1, in a drilldown according to Customer, all data from InfoCube 1 is displayed under initial value # (not assigned). The query would look as follows on the MultiProvider:
we can use the constant selection option of Bex Query to solve above not assigned problems.
In the Query Designer, define a selection that contains key figure "Amount" and characteristic "Customer" and Restrict characteristic "Customer" to initial value # (not assigned) and set the constant selection on characteristic Customer
In the drilldown, the rows with initial value # (not assigned) on characteristic Customer are grayed out, but the data about key figure Amount is retained.
The query looks like this: