You would probably want to use something like what is described in the Complex Output section of this page: https://oracle-base.com/articles/misc/oracle-rest-data-services-ords-restful-web-services-handling-complex-json-payloads…
Using the CURSOR keyword within your SELECT will allow you to create nested arrays within your JSON response.
looks like something you could do with a cursor
select werb_id, somedescriptions, cursor ( select art_nr, var, vk_preis_eur
from table b
where a.werb_id = b.werb_id ) werbs
from werbs a;
just set your pagesize to 0