MySQL Select parents 和孩子以正确的顺序与单个查询

Question

我有一个 MySQL table 具有以下数据：

ID Name      ParentID
1  Foo       null
2  Bar       null
3  Foo SubA  1
4  Bar SubA  2
5  Foo SubC  1
6  Foo SubB  1

我想按以下顺序检索所有数据：

1  Foo       null
3  Foo SubA  1
6  Foo SubB  1
5  Foo SubC  1
2  Bar       null
4  Bar SubA  2

是否可以使用 MySQL 和单个查询？

Answer 1

您可以使用递归 CTE (MySQL 8.0+):

-- 2 level hierarchy (parent-child)
WITH RECURSIVE cte AS
(
   SELECT tx.*, 1 AS lvl, ID AS grp FROM tx WHERE ParentID IS NULL
   UNION ALL 
   SELECT tx.*, lvl+1, cte.ID FROM tx JOIN cte WHERE tx.ParentId = cte.Id
)
SELECT ID, Name, ParentId
FROM cte
ORDER BY grp, lvl, Name;

DBFiddle Demo

Answer 2

如果这是一个 two-level 层次结构，即没有 grandparents 和 grandchildren，它只是一个 ORDER BY 子句：

select id, name, parentid
from mytable
order by coalesce(parentid, id), parentid is not null, name;

这利用了 MySQL 的 true = 1，false = 0。parentid is not null 对于 parent 是 0，对于 children 是 1。

MySQL Select parents 和孩子以正确的顺序与单个查询

MySQL Select parents and childs in proper order with single query

mysql

sql

mysql-5.6