问题提出：
一个高级SQL语句问题
假设有一张表，有这样一些数据
a b
1 2
0 2
2 3
3 4
4
5 6
6 7
12 7
7
a是子节点 b是父节点 ,如果b为null 则为一棵树的根节点
根据A与B的这个关系 找出一棵树的结构显示出来 ，并显示出每个节点在书中的层级，将树的结构打印出来
我这边假设树最多有5个层级，SQL如下：
with tt as(
select 1 a,2 b from dual
union all SELECT 0,2 from dual
union all select 2,3 from dual
union all select 3,4 from dual
union all select 4,null from dual
union all select 5,6 from dual
union all select 6,7 from dual
union all select 12,7 from dual
union all select 7,null from dual)
SELECT
a
,tree_lvl
,root
,tree_lvl_path
,(CASE
WHEN tree_lvl >1 THEN SUBSTR(tree_lvl_path,1,instr(tree_lvl_path,',',1,1)-1 )
ELSE tree_lvl_path
END) COL01
,(CASE
WHEN tree_lvl <2 THEN NULL
WHEN tree_lvl =2 THEN SUBSTR(tree_lvl_path,instr(tree_lvl_path,',',1,1)+1 )
WHEN tree_lvl >2 THEN SUBSTR(tree_lvl_path,instr(tree_lvl_path,',',1,1)+1,instr(tree_lvl_path,',',1,2)- instr(tree_lvl_path,',',1,1)-1 )
END) COL02
,(CASE
WHEN tree_lvl <3 THEN NULL
WHEN tree_lvl =3 THEN SUBSTR(tree_lvl_path,instr(tree_lvl_path,',',1,2)+1 )
WHEN tree_lvl >3 THEN SUBSTR(tree_lvl_path,instr(tree_lvl_path,',',1,2)+1,instr(tree_lvl_path,',',1,2)- instr(tree_lvl_path,',',1,1)-1 )
END) COL03
,(CASE
WHEN tree_lvl <4 THEN NULL
WHEN tree_lvl =4 THEN SUBSTR(tree_lvl_path,instr(tree_lvl_path,',',1,3)+1 )
WHEN tree_lvl >4 THEN SUBSTR(tree_lvl_path,instr(tree_lvl_path,',',1,3)+1,instr(tree_lvl_path,',',1,3)- instr(tree_lvl_path,',',1,2)-1 )
END) COL04
,(CASE
WHEN tree_lvl <5 THEN NULL
WHEN tree_lvl =5 THEN SUBSTR(tree_lvl_path,instr(tree_lvl_path,',',1,4)+1 )
WHEN tree_lvl >5 THEN SUBSTR(tree_lvl_path,instr(tree_lvl_path,',',1,4)+1,instr(tree_lvl_path,',',1,5)- instr(tree_lv
启动“OracleDBConsole”时－－－出现“Agent process exited abnormally during initialization.”
在运行"emctl start dbconsole " 发现是找不到指定文件,在网上找了一下原因, 更改的指定文件夹的名称，但还是不能启动服务，结果认真看了一下，是CTRL+C,CTRL+V 然后才启动成功，所以特定写下来，记 ......
