connect-by-prior：使用 PRIOR 向上钻取 2 个级别

Question

我有以下先验连接 SQL，它基本上从叶节点开始，沿着树向上到达父级“tree-trunc”（级别 1）：

with my_tree as (
   select 'level 4.1' node, 'level 3.1' parent_node from dual union
   select 'level 4.2' node, 'level 3.2' parent_node from dual union
   select 'level 3.1' node, 'level 2'   parent_node from dual union
   select 'level 3.2' node, 'level 2'   parent_node from dual union
   select 'level 2'   node, 'level 1'   parent_node from dual union
   select 'level 1'   node, ''          parent_node from dual
)
select level, t.node, t.parent_node, prior t.node child_that_pointed_me_here
from   my_tree t
connect by t.node = prior t.parent_node
start with t.node like 'level 4%'

它给了我以下输出：

level node      parent_node child_who_pointed_me_here
----- --------- ----------- -------------------------
1     level 4.1 level 3.1 
2     level 3.1 level 2     level 4.1
3     level 2   level 1     level 3.1
4     level 1               level 2
1     level 4.2 level 3.2 
2     level 3.2 level 2     level 4.2
3     level 2   level 1     level 3.2
4     level 1               level 2

您可以看到指令prior t.node（别名为列child_who_pointed_me_here）将我带到“先前”记录中的数据（即我开始的子节点），这正是我想要的。换句话说，PRIOR 关键字让我可以访问“上一个”记录中的数据。

但我想要的是访问之前的 2 级（或 3 或 4 级）数据。像prior prior t.node 这样的东西。所需的输出如下所示：

level node      parent_node child_who_pointed_me_here grandchild_who_pointed_me_here
----- --------- ----------- ------------------------- ------------------------------
1     level 4.1 level 3.1 
2     level 3.1 level 2     level 4.1
3     level 2   level 1     level 3.1                 level 4.1
4     level 1               level 2                   level 3.1
1     level 4.2 level 3.2
2     level 3.2 level 2     level 4.2
3     level 2   level 1     level 3.2                 level 4.2
4     level 1               level 2                   level 3.2

我已经尝试了明显的prior prior t.node，但它显然只会导致不支持的语法类型错误。

因此的问题是：connect-by SQL 中是否有一个构造可以让我沿着路径返回（或向上）2 个级别？

我正在使用 Oracle 12c 来构建这个 SQL，但任何 SQL 风格的答案都值得赞赏。

Answer 1

sys_connect_by_path 将为您提供您到达当前行的整个沿袭。

我通过反转和选择第二个“/”和第三个“/”的索引位置来对 sys_connect_by_path 执行字符串操作，以提取其间的数据

使用此字符串，我们可以使用以下内容提取 2 层，如下所示。

with my_tree as (
   select 'level 4.1' node, 'level 3.1' parent_node from dual union
   select 'level 4.2' node, 'level 3.2' parent_node from dual union
   select 'level 3.1' node, 'level 2'   parent_node from dual union
   select 'level 3.2' node, 'level 2'   parent_node from dual union
   select 'level 2'   node, 'level 1'   parent_node from dual union
   select 'level 1'   node, ''          parent_node from dual
)
select level, t.node, t.parent_node, prior t.node child_that_pointed_me_here
       ,sys_connect_by_path(t.node,'/') as lineage
       ,rtrim(
        reverse(
         substr(
               reverse(sys_connect_by_path(t.node,'/'))
               ,instr(reverse(sys_connect_by_path(t.node,'/')),'/',1,2)
               ,instr(reverse(sys_connect_by_path(t.node,'/')),'/',1,3)
               -
               instr(reverse(sys_connect_by_path(t.node,'/')),'/',1,2)
               )
              )
        ,'/') as two_level_up
  from my_tree t
connect by t.node = prior t.parent_node
start with t.node like 'level 4%'



+-------+-----------+-------------+----------------------------+--------------------------------------+--------------+
| LEVEL |   NODE    | PARENT_NODE | CHILD_THAT_POINTED_ME_HERE |               LINEAGE                | TWO_LEVEL_UP |
+-------+-----------+-------------+----------------------------+--------------------------------------+--------------+
|     1 | level 4.1 | level 3.1   | null                       | /level 4.1                           |              |
|     2 | level 3.1 | level 2     | level 4.1                  | /level 4.1/level 3.1                 |              |
|     3 | level 2   | level 1     | level 3.1                  | /level 4.1/level 3.1/level 2         | level 4.1    |
|     4 | level 1   |             | level 2                    | /level 4.1/level 3.1/level 2/level 1 | level 3.1    |
|     1 | level 4.2 | level 3.2   |                            | /level 4.2                           |              |
|     2 | level 3.2 | level 2     | level 4.2                  | /level 4.2/level 3.2                 |              |
|     3 | level 2   | level 1     | level 3.2                  | /level 4.2/level 3.2/level 2         | level 4.2    |
|     4 | level 1   |             | level 2                    | /level 4.2/level 3.2/level 2/level 1 | level 3.2    |
+-------+-----------+-------------+----------------------------+--------------------------------------+--------------+

dbfiddle https://dbfiddle.uk/?rdbms=oracle_11.2&fiddle=0448131cc387e52eab3126dfce0a7cde