@Alex @PaCo
I am trying to select all the people that have filled the postal code [rcs300] from address of contact, and empty the Municipality [rsc280].
Like this:

It returns NO record.
If I change it and put a postal code (that I know that has the municipality empty), like this:

It return some records (in this case 27).
It seems illogical since these records should have been selected in the first selection.
Am I doing somethig wrong that I do not detect?
Thank you in advance!