SYS_CONNECT_BY_PATH

/*
等价,相当于把每一条记录做为start with 来建立树
必须有connect by,可以没有start with
*/

select Cow,
       Bull,
       LPAD(' ', 6 * (Level - 1)) || Offspring Offspring,
       Sex,
       Birthdate,
       level
  from BREEDING
 --start with Offspring = 'EVE'
connect by Cow = PRIOR Offspring;

select Cow,
       Bull,
       LPAD(' ', 6 * (Level - 1)) || Offspring Offspring,
       Sex,
       Birthdate,
       level
  from BREEDING
 start with Offspring in (select offspring from BREEDING)
connect by Cow = PRIOR Offspring;
/*
Cow = PRIOR Offspring;
又可以这样理解,start with 的记录的offspring 字段= 哪些记录的cow,列出那些记录
*/


代码
SELECT ename   
FROM scott.emp    
START WITH ename = 'KING'    
CONNECT BY PRIOR empno = mgr;    
  
--得到结果为：   
  
KING   
JONES   
SCOTT   
ADAMS   
FORD   
SMITH   
BLAKE   
ALLEN   
WARD   
MARTIN   
TURNER   
JAMES   
  
  
  
而：   
  
  
  
SELECT SYS_CONNECT_BY_PATH(ename, '>') "Path"    
FROM scott.emp    
START WITH ename = 'KING'    
CONNECT BY PRIOR empno = mgr;   
  
  
  
--得到结果为：   
  
  
  
>KING   
>KING>JONES   
>KING>JONES>SCOTT   
>KING>JONES>SCOTT>ADAMS   
>KING>JONES>FORD   
>KING>JONES>FORD>SMITH   
>KING>BLAKE   
>KING>BLAKE>ALLEN   
>KING>BLAKE>WARD   
>KING>BLAKE>MARTIN   
>KING>BLAKE>TURNER   
>KING>BLAKE>JAMES   
>KING>CLARK   
>KING>CLARK>MILLER  
SELECT ename
FROM scott.emp 
START WITH ename = 'KING' 
CONNECT BY PRIOR empno = mgr; 

--得到结果为：

KING
JONES
SCOTT
ADAMS
FORD
SMITH
BLAKE
ALLEN
WARD
MARTIN
TURNER
JAMES

 

而：

 

SELECT SYS_CONNECT_BY_PATH(ename, '>') "Path" 
FROM scott.emp 
START WITH ename = 'KING' 
CONNECT BY PRIOR empno = mgr;

 

--得到结果为：

 

>KING
>KING>JONES
>KING>JONES>SCOTT
>KING>JONES>SCOTT>ADAMS
>KING>JONES>FORD
>KING>JONES>FORD>SMITH
>KING>BLAKE
>KING>BLAKE>ALLEN
>KING>BLAKE>WARD
>KING>BLAKE>MARTIN
>KING>BLAKE>TURNER
>KING>BLAKE>JAMES
>KING>CLARK
>KING>CLARK>MILLER
 


其实SYS_CONNECT_BY_PATH这个函数是oracle9i才新提出来的！
它一定要和connect by子句合用！
第一个参数是形成树形式的字段，第二个参数是父级和其子级分隔显示用的分隔符！

START WITH 代表你要开始遍历的的节点！

CONNECT BY PRIOR 是标示父子关系的对应！

如下例子：

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select max(   
substr(   
sys_connect_by_path(column_name,',')   
,2)   
)   
from (select column_name,rownum rn from user_tab_columns where table_name ='AA_TEST')   
start with rn=1 connect by rn=rownum ;  
select max(
substr(
sys_connect_by_path(column_name,',')
,2)
)
from (select column_name,rownum rn from user_tab_columns where table_name ='AA_TEST')
start with rn=1 connect by rn=rownum ;

 

是将列用,进行分割成为一行，然后将首个，去掉，只取取最大的那个数据。

－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－

下面是别人的例子：

1、带层次关系

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SQL> create table dept(deptno number,deptname varchar2(20),mgrno number);   
  
Table created.   
  
SQL> insert into dept values(1,'总公司',null);   
  
1 row created.   
  
SQL> insert into dept values(2,'浙江分公司',1);   
  
1 row created.   
  
SQL> insert into dept values(3,'杭州分公司',2);   
  
1 row created.   
  
SQL> commit;   
  
Commit complete.   
  
SQL> select max(substr(sys_connect_by_path(deptname,','),2)) from dept connect by prior deptno=mgrno;   
  
MAX(SUBSTR(SYS_CONNECT_BY_PATH(DEPTNAME,','),2))   
--------------------------------------------------------------------------------   
总公司,浙江分公司,杭州分公司  
SQL> create table dept(deptno number,deptname varchar2(20),mgrno number);

Table created.

SQL> insert into dept values(1,'总公司',null);

1 row created.

SQL> insert into dept values(2,'浙江分公司',1);

1 row created.

SQL> insert into dept values(3,'杭州分公司',2);

1 row created.

SQL> commit;

Commit complete.

SQL> select max(substr(sys_connect_by_path(deptname,','),2)) from dept connect by prior deptno=mgrno;

MAX(SUBSTR(SYS_CONNECT_BY_PATH(DEPTNAME,','),2))
--------------------------------------------------------------------------------
总公司,浙江分公司,杭州分公司 

2、行列转换


如把一个表的所有列连成一行，用逗号分隔:

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SQL> select max(substr(sys_connect_by_path(column_name,','),2))   
from (select column_name,rownum rn from user_tab_columns where table_name ='DEPT')   
start with rn=1 connect by rn=rownum ;   
  
MAX(SUBSTR(SYS_CONNECT_BY_PATH(COLUMN_NAME,','),2))   
--------------------------------------------------------------------------------   
DEPTNO,DEPTNAME,MGRNO  

 
 

 

 select SYS_CONNECT_BY_PATH(DEPTNAME, '<'), DEPTNAME, deptno, mgrno, level
   from dept
 connect by prior deptno = mgrno;
 --return 6 records

 --MAX函数只是一个巧合按照字幕排列,我们应该是max(levle),或者长度最长
 
 select MAX(SUBSTR(SYS_CONNECT_BY_PATH(DEPTNAME, '<'), 2))
   from dept
 connect by prior deptno = mgrno;