Question

我有这样的层次结构数据：

Id, Related
1, 0
2, 1
3, 1
4, 1
5, 4
6, 3
7, 0
8, 7
9, 3
10, 8

我想选择2个父母，然后为每个父母找到2个孩子

结果：

类似的东西：

select * from TABLE T1 where T1.related=0 order by T2.id limit 2 
union 
select * from TABLE T2 where T1.id=T2.related order by T2.id limit 2

我一直在研究mysql选择分层数据，但我找不到任何可以帮助我的情况。

Answer 1

试试这个。我在MSSQl中测试它。我希望它也适用于mysql

SELECT id,child FROM
(SELECT t1.Related id,t1.id child,row_number() over (partition by t1.Related ORDER BY t1.Id) sl FROM Table1 t1
INNER JOIN Table1 t2 ON t1.Related=t2.Id) t
where t.sl<=2

OR

SET @row_number:=0;
SET @Related:=0;
SET @Child:=0;
SELECT t.id,t.Child FROM 
(SELECT @row_number:=CASE WHEN @Related=t1.Related THEN @row_number+1 ELSE 1 END AS row_nu, @Related:=t1.Related AS id,@Child:=t1.id AS Child
FROM Table1 t1
INNER JOIN Table1 t2 ON t1.Related=t2.Id) t
WHERE t.row_nu<=2;

DB Fiddle upadte DB Fiddle

Answer 2

您想要选择Related = 0的所有记录。然后，您要选择最多两个随机孩子。一种解决方案是获取最小子ID和最大子ID。您可以在子查询中查找父项。

select
  related as parent_id, 
  min(id) as child1_id,
  max(id) as child2_id
from mytable
where related in (select id from mytable where related = 0)
group by related;

如果您不想两次显示相同的子ID（如果只有一个孩子，即min(id) = max(id)），请将其更改为：

select
  related as parent_id, 
  min(id) as child1_id,
  nullif(max(id), min(id)) as child2_id
...

这些查询没有显示没有孩子的related = 0记录，但是，如果没有孩子，他们不是父母： - ）

MySQL使用union选择层次结构

2 个答案: