分层查询以显示每个父级下的所有子级

Question

请参阅所附图片：

树的可视化图

所需的查询输出

例如，这是数据结构，如图像（图）所示。我需要一个查询，该查询将在第二张图像中显示相同的结果（以显示每个父级下的所有子级）。

下面是我尝试过的代码（结果与我所寻找的不一样）：

 SELECT parent,
        LEVEL,
        child

FROM  table1 

CONNECT BY NOCYCLE parent = PRIOR child;

Answer 1

我将表中的列名分别从“ CHILD”和“ PARENT”更改为“ ID”和“ PARENT_ID”，以减少混乱。

无论如何，您需要使用 CONNECT_BY_ROOT(parent_id) 来获得所需输出中显示为“ PARENT”的内容。如果您不关心结果中的行顺序，则只需：

SELECT connect_by_root(parent_id) "PARENT", id "CHILD"
FROM   table1
WHERE  connect_by_root(parent_id) is not null
CONNECT BY parent_id = prior id

如果您确实关心行的顺序，则会变得更加困难。您需要检查树中每个节点的深度，并使用该深度对结果进行排序。那应该是：

with hier as ( 
SELECT connect_by_root(parent_id) root_id, id, level, 
       case when connect_by_root(parent_id) is null then level else null end root_depth
FROM   table1
CONNECT BY parent_id = prior id
)
select h1.root_id "PARENT", h1.id "CHILD"
from hier h1 inner join hier h2 on h2.root_id is null and h2.id = h1.root_id
order by h2.root_depth, h1.root_id, h1.id

这是一个完整的示例，其中包含测试数据：

WITH table1 ( id, parent_id ) AS
( SELECT 'A', null FROM DUAL UNION ALL
  SELECT 'A1', 'A' FROM DUAL UNION ALL
  SELECT 'A2', 'A' FROM DUAL UNION ALL
  SELECT 'A3', 'A' FROM DUAL UNION ALL
  SELECT 'A11', 'A1' FROM DUAL UNION ALL
  SELECT 'A12', 'A1' FROM DUAL UNION ALL
  SELECT 'A21', 'A2' FROM DUAL UNION ALL
  SELECT 'A121', 'A12' FROM DUAL UNION ALL
  SELECT 'A122', 'A12' FROM DUAL ),
-- Solution begins here 
hier as ( 
SELECT connect_by_root(parent_id) root_id, id, level lvl, 
       case when connect_by_root(parent_id) is null then level else null end root_depth
FROM   table1
CONNECT BY parent_id = prior id
)
select h1.root_id "PARENT", h1.id "CHILD", h2.*
from hier h1 inner join hier h2 on h2.root_id is null and h2.id = h1.root_id
order by h2.root_depth, h1.root_id, h1.lvl, h1.id

+--------+-------+
| PARENT | CHILD |
+--------+-------+
| A      | A1    |
| A      | A2    |
| A      | A3    |
| A      | A11   |
| A      | A12   |
| A      | A21   |
| A      | A121  |
| A      | A122  |
| A1     | A11   |
| A1     | A12   |
| A1     | A121  |
| A1     | A122  |
| A2     | A21   |
| A12    | A121  |
| A12    | A122  |
+--------+-------+