[Koha] Reports: how to query MARC subfield 946a

Dear all, We are working on koha 3.0.3 and would like to create a report within koha that includes the MARC Subfield 946a, in order to get a table with something like author, title, ... and the content of field 946a. How could we query this value? In which table.field can it be found? Appreciating any hints. Best, Marc