sql - 在 Postgres 中订购 WITH RECURSIVE 查询

Question

我正在 Postgres 中执行递归查询以检索电子邮件列表及其线程子级，如下所示:

WITH RECURSIVE cte (id, title, path, parent_id, depth)  AS (
  SELECT  id, 
          title,
          array[id] AS path,
          parent_id, 
          1 AS depth
  FROM    emails
  WHERE   parent_id IS NULL

  UNION ALL

  SELECT  emails.id,
          emails.title,
          cte.path || emails.id,
          emails.parent_id, 
          cte.depth + 1 AS depth
  FROM    emails
          JOIN cte ON emails.parent_id = cte.id
)
SELECT id, title, path, parent_id, depth FROM cte
ORDER BY path;

在找到子邮件之前，如何更改列表的顺序(例如按标题排序)。我显然需要保留外部 ORDER BY，以便按树顺序检索列表，而 Postgres 不允许我在 UNION ALL 之前插入 ORDER BY 子句。

谢谢，

Answer 1

这是未经测试的，但通常我可以在联合之前添加任何 ORDER BY，只要有括号...

WITH RECURSIVE cte (id, title, path, parent_id, depth)  AS (
(  SELECT  id, 
          title,
          array[id] AS path,
          parent_id, 
          1 AS depth
  FROM    emails
  WHERE   parent_id IS NULL
  ORDER BY title
)
  UNION ALL

  SELECT  emails.id,
          emails.title,
          cte.path || emails.id,
          emails.parent_id, 
          cte.depth + 1 AS depth
  FROM    emails
          JOIN cte ON emails.parent_id = cte.id
)
SELECT id, title, path, parent_id, depth FROM cte
ORDER BY path;