Oracle子查询没有从外部块2级别上看到该变量 [英] Oracle subquery does not see the variable from the outer block 2 levels up

问题描述

我想在一个查询中获得帖子以及与该帖子相关联的第一条评论.这是我在PostgreSQL中的操作方式:

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  

它工作正常.

但是，在Oracle中出现错误ORA-00904 p.post_id:无效的标识符.

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

对于一个子选择似乎很好用，但是由于我需要使用rownum(在Oracle中没有限制/偏移量)这一事实，我无法仅获得一个注释.

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).

我在做什么错了?

推荐答案

不，Oracle不与嵌套多于一个深度的子查询相关(MySQL也不相关).

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

这是一个众所周知的问题.

This is a well-known problem.

使用此:

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

这篇关于Oracle子查询没有从外部块2级别上看到该变量的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！