SQL父/子CTE排序 [英] SQL Parent/Child CTE Ordering

问题描述

我正在尝试创建一个论坛设置，您可以在其中对特定帖子发表评论，并使回复按日期顺序显示在父级下方。父级下面只有一个子级别。这是示例数据集：

I'm trying to create a forum setup where you can comment on specific posts and have the replies show immediately below the parent in date order. There will only be one sub-level below the parent. Here's the example dataset:

ID.ParentID.Datestamp
12.NULL.2013-03-01 1:00pm
13.NULL.2013-03-01 2:00pm
14.12.2013-03-01 2:20pm
15.12.2013-03-01 2:30pm
16.NULL.2013-03-01 3:30pm

这就是我要结束的事情：

And here's what I'm trying to end up with:

12.NULL.2013-03-01 1:00pm
14.12.2013-03-01 2:20pm
15.12.2013-03-01 2:30pm
13.NULL.2013-03-01 2:00pm
16.NULL.2013-03-01 3:30pm

我知道我需要某种CTE，但这并不能使孩子在适当的父母的陪同下（显然，因为没有ORDER BY子句）；我找不到正确的顺序。谁能提供一些见识？

I know I need some sort of CTE going, but this doesn't order the children under the appropriate parent (obviously, since there's no ORDER BY clause); I couldn't figure out the proper ordering. Can anyone provide some insight?

; WITH Messages
AS 
(
    SELECT ID, ParentID, Datestamp
    FROM ForumMessages
    WHERE ParentID IS NULL

    -- Recursive
    UNION ALL 
    SELECT
        t2.ID, t2.ParentID, t2.Datestamp
    FROM
        ForumMessages AS t2
        JOIN Messages AS m ON t2.ParentID = m.ID
)

SELECT ID, ParentID, Datestamp
FROM Messages

推荐答案

对于单个级别的深度，您无需使用递归-尝试：

For a single level depth, you don't need to use recursion - try:

SELECT ID, ParentID, Datestamp
FROM ForumMessages
order by coalesce(ParentID,ID), Datestamp

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