Как сделать такую выборку в oracle sql?

select a.first_letter_fam, a.avg_plateg_by_letter
from (select substr(n.c_last_name,1,1) as first_letter_fam, avg(p.n_sum) over (partition by substr(n.c_last_name,1,1)) as avg_plateg_by_letter, (n_info_cold + n_info_hot) as potr_vody, avg(n_info_cold + n_info_hot) over () as avg_potr_vody_all
from computation as c, client as n, payment as p
where c.n_client = n.n_client) as a
where a.potr_vody < a.avg_potr_vody_all

Получилось вот так. внутренний подзапрос выходит с ошибкой ORA-00933: SQL command not properly ended

Весь запрос полностью с ошибкой ORA-00907: missing right parenthesis