Thanks again David. I am very novice, and tried the following which does not give an error but returns null instead of []
select
json_agg(
(select row_to_json(_) from
(select
vc.id, vc.descr, vc.expense, vc.version, vat_percentage) as _
)
)
from vat_cat vc
left join (
select
vatcat_id,
COALESCE(NULLIF(
json_agg(
(select row_to_json(_) from
(select vp.percentage, vp.version, json_build_object(
'lower', lower(vp.validity),
'upper', upper(vp.validity),
'lower_inc', lower_inc(vp.validity),
'upper_inc', upper_inc(vp.validity)
) validity)
as _)
)::text,
'null'::text), '[]'::text) vat_percentage
from vat_percentage vp
group by vatcat_id
) vp on
vc.id = vp.vatcat_id