SQL如何按三列排序，但在树状结构中

Question

数据库是mariadb。这是我的表格，我想按“id”降序对其进行排序，但子项始终位于父项下方。

+----+-----------+--------------+--------+----------+
| id | firstName | favoriteFood | status | parentID |
+----+-----------+--------------+--------+----------+
| 1  |   Andy    |    pizza     | child  |    5     |
| 2  |   Alice   |    burger    | child  |    5     |
| 3  |    Bob    |    fries     | orphan |   null   |
| 4  |  Barney   |    salad     | parent |   null   |
| 5  |  Christ   |    steak     | parent |   null   |
| 6  |  Daniel   |    pizza     | child  |    8     |
| 7  |   Mike    |    fries     | child  |    5     |
| 8  |  Richard  |   oatmeal    | parent |   null   |
| 9  |  Wilson   |    steak     | child  |    8     |
| 10 |   Dicky   |  watermelon  | orphan |   null   |
| 11 |   Freya   |    potato    | orphan |   null   |
| 12 |   Ryan    |    oyster    | parent |   null   |
| 13 |   Alex    |    bread     | orphan |   null   |
| 14 |   Sarah   |   brocoli    | child  |    12    |
| 15 |   Dane    |    toast     | child  |    8     |
+----+-----------+--------------+--------+----------+

我尝试过这个，但它没有给出我预期的结果

SELECT * from table ORDER BY id DESC, FIELD(status,'parent','child'), parentID

如果可能的话，这是我的预期结果：

+----+-----------+--------------+--------+----------+
| id | firstName | favoriteFood | status | parentID |
+----+-----------+--------------+--------+----------+
| 13 |   Alex    |    bread     | orphan |   null   |
| 12 |   Ryan    |    oyster    | parent |   null   |
| 14 |   Sarah   |   brocoli    | child  |    12    |
| 11 |   Freya   |    potato    | orphan |   null   |
| 10 |   Dicky   |  watermelon  | orphan |   null   |
| 8  |  Richard  |   oatmeal    | parent |   null   |
| 15 |   Dane    |    toast     | child  |    8     |
| 9  |  Wilson   |    steak     | child  |    8     |
| 6  |  Daniel   |    pizza     | child  |    8     |
| 5  |  Christ   |    steak     | parent |   null   |
| 7  |   Mike    |    fries     | child  |    5     |
| 2  |   Alice   |    burger    | child  |    5     |
| 1  |   Andy    |    pizza     | child  |    5     |
| 4  |  Barney   |    salad     | parent |   null   |
| 3  |    Bob    |    fries     | orphan |   null   |
+----+-----------+--------------+--------+----------+

Answer 1

您可以使用

recursive common table expression

来遍历层次树，如下所示：

with recursive CTE AS (
   select id, firstName, favoriteFood, status, parentID, 1 as Lvl, id as gParentId
   from MyTbl
   where parentID is null
   union all
   select m.id, m.firstName, m.favoriteFood, m.status, m.parentID, Lvl+1 as Lvl, c.gParentId as gParentId
   from MyTbl m
        inner JOIN 
        CTE c on c.Id=m.ParentId
) 
select  * 
from CTE 
order by gParentId desc, lvl, id desc

我们从那些parentId=null 的行开始：这些行位于（颠倒树的）根级别，而不是在每次迭代时我们检索其子项、孙子项等。我们还生成迭代计数 (Lvl)，并跟踪根“gParentId”，以便我们对它们进行排序。

我无法访问 MariaDB，因此我在 SQLServer 中对此进行了测试，但它应该可以与添加该递归关键字一起使用。