Get multiple ID columns into one column in a matrix

Greetings everyone, 

I want to combine multiple (two) columns that have parent/child relationship into one column when drilling down in a matrix. Let me give an example - the matrix looks like this now:

So I want a column name "ID" combined of the other two columns (Program ID and Phase ID) with the following conditions:

- For program it only shows the program ID

- When drilling down to the phases it shows the phase IDs

- and for the manager name it should show a blank value because there is no ID for it

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So the result should look like this:

The data is comming from two tables: Programs and Phases;

There is a "one-to-many" relationship between the two tables (Program > Phase)

I appreciate any help regarding this issue and thanks in advance!