user248789 user248789 - 2 years ago 221
SQL Question

Oracle subquery does not see the variable from the outer block 2 levels up

I'd like to get in one query a post and the first comment associated with the post. Here is how I do it in PostgreSQL:

SELECT p.post_id,
(select * from
(select comment_body from comments where post_id = p.post_id
order by created_date asc) where rownum=1
) the_first_comment
FROM posts p

and it works fine.

However, in Oracle I'm getting an error ORA-00904 p.post_id: invalid identifier.

It seems to work fine for one subselect, but I cannot get the comment with only one due to the fact that I need to use rownum (no limit / offset in Oracle).

What am I doing wrong here?

Answer Source

No, Oracle doesn't correlate the subqueries nested more than one level deep (and neither does MySQL).

This is a well-known problem.

Use this:

SELECT  p.post_id, c.*
FROM    posts
JOIN    (
        SELECT  c.*, ROW_NUMBER() OVER (PARTITION BY post_id ORDER BY created_date ASC) AS rn
        FROM    comments c
        ) c
ON      c.post_id = p.post_id
        AND rn = 1
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