Extrating XML whilst using group by: ORA-22806

Question

Hi,
Can anyone tell me why this works:
select media_id, count(*) from (
select m.media_id, m.mediadata.extract('//med:caption/text()|//med:title/text()').getStringVal() AS title_caption
from media m
where m.media_id = 3208
group by media_id;
and this works:
select title_caption
from (
select m.mediadata.extract('//med:caption/text()|//med:title/text()').getStringVal() AS title_caption
from media m
where m.media_id = 3208
but this doesn’t?
select title_caption, count(*) from (
select m.media_id, m.mediadata.extract('//med:caption/text()|//med:title/text()').getStringVal() AS title_caption
from media m
where m.media_id = 3208
group by title_caption;
select title_caption, count(*) from (
ERROR at line 1:
ORA-22806: not an object or REF
mediadata is a column of type xmltype and media_id is a number.
Thanks in advance.

Answer 1

I haven't fixed this problem but I've found a workaround, creating a view that performs the extract. The view then contains a varchar2 which can be grouped by.