Oracle newbie error: ORA-00904 Invalid identifier when using "case when" - oracle

Question

I'm getting an error in a query. This query is OK and returning data (the selects and rownums are for pagination):
select *
from (select a.*, rownum rnum
from (select id_edition, id_document, name, extension, creation_date, url,
(select inscription_date from edition_student_d0 where id_edition = 12345 and id_third = 12345) inscription_date
from upd_edition_doc_d0
where id_edition = 1071591
order by creation_date desc) a
where rownum <= 10 )
where rnum >= 1
Now I'm trying to include a "case when" and get the url only in some circumstances, so I make these modifications, including a case block:
select *
from (select a.*, rownum rnum
from (select id_edition, id_document, name, extension, creation_date,
(select inscription_date from edition_student_d0 where id_edition = 12345 and id_third = 12345) inscription_date,
case
when trunc(inscription_date + 90) <= trunc(sysdate) then null
else url
end url
from upd_edition_doc_d0
where id_edition = 1071591
order by creation_date desc) a
where rownum <= 10 )
where rnum >= 1
Oracle launches this error:
ORA-00904: "INSCRIPTION_DATE": invalid identifier
I suppose that's because I'm asking for inscription_date and using it at the same query level, but I don't know how to deal with this.
Also, how can I make what I'm trying to make? I mean, getting url only under a certain condition.
Anybody can help?
Thank you in advance.

Answer 1

You can't refer an alias in the same level of the query.
You can replace the subquery...
select *
from (select a.*, rownum rnum
from (select id_edition, id_document, name, extension, creation_date,
(select inscription_date from edition_student_d0 where id_edition = 12345 and id_third = 12345) inscription_date,
case when trunc((select inscription_date from edition_student_d0 where id_edition = 12345 and id_third = 12345) + 90) <= trunc(sysdate) then null
else url
end as url
from upd_edition_doc_d0
where id_edition = 1071591
order by creation_date desc) a
where rownum <= 10 )
where rnum >= 1
OR move the case one level up.
select *
from (select a.*,
case when trunc(inscription_date + 90) <= trunc(sysdate) then null
else url
end as url,
rownum rnum
from (select id_edition, id_document, name, extension, creation_date,
(select inscription_date from edition_student_d0 where id_edition = 12345 and id_third = 12345) inscription_date
from upd_edition_doc_d0
where id_edition = 1071591
order by creation_date desc) a
where rownum <= 10 )
where rnum >= 1