PostgreSQL嵌套CTE和UNION

我正在尝试学习sql，使用Postgresql 9.1.3。我想了解一些令我震惊的行为是不一致的。以机智：

这样做：

WITH innermost AS (SELECT 2)
SELECT * FROM innermost
UNION SELECT 3;

我得到这个：

?column? 
----------
        2
        3

这样做：

WITH outmost AS (
        (WITH innermost AS (SELECT 2)
         SELECT * FROM innermost)
)                                
SELECT * FROM outmost;

结果：

?column? 
----------
        2

这也可以：

WITH outmost AS (
  SELECT 1
  UNION (WITH innermost AS (SELECT 2)
         SELECT * FROM innermost)
)
SELECT * FROM outmost;

我得到这个：

?column? 
----------
        1
        2

但这不行：

WITH outmost AS (
  SELECT 1
  UNION (WITH innermost as (SELECT 2)
         SELECT * FROM innermost
         UNION SELECT 3)
)
SELECT * FROM outmost;

结果：

ERROR:  relation "innermost" does not exist
LINE 4:          SELECT * FROM innermost

对于我的思维方式，最后一个应该成功，还有一个应该失败。我看不到图案。有没有一些一般规则，可以使我预测嵌套CTE和UNION的组合将会或不会起作用？

这个奥秘解决了：我所观察到的行为是一个已知的错误。我将相同的原始帖子发送到Postgresql特定的列表，并得到这个答案：

This is a bug :-(. The parse analysis code seems to think that WITH
can only be attached to the top level or a leaf-level SELECT within a
set operation tree; but the grammar follows the sql standard which
says no such thing. The WITH gets accepted,and attached to the
intermediate-level UNION which is where syntactically it should go,
and then it’s entirely ignored during parse analysis. Will see about
fixing it.

06000

http://archives.postgresql.org/pgsql-novice/2012-07/msg00113.php

原文地址：https://www.jb51.cc/postgresql/192913.html