[Koha] loop over subjects (in SQL report)

Hi, I would like to list records by subject (in a SQL report). I can isolate the first subject in a record with ExtractValue(biblioitems.marcxml,'//datafield[substring(@tag,1,1) = "6"][1]/subfield[@code>="a"]') But then I need to perform a UNION DISTINCT with another SELECT addressing the second subject (with 2 replacing 1 between square brackets above), etc, when records can have a much larger number of subjects (MARC field starting with 6). Any suggestion? Thanks, Giuseppe.