概述
层次化查询,即树型结构查询,是SQL中经常用到的功能之一,通常由根节点,父节点,子节点,叶节点组成,其语法如下:
SELECT [LEVEL] ,column,expression,...
FROM table_name
[WHERE where_clause]
[[START WITH start_condition] [CONNECT BY PRIOR prior_condition]];
LEVEL:伪列,用于表示树的层次
start_condition：层次化查询的起始条件，指定阶层的根。
prior_condition：定义父节点和子节点之间的关系，PRIOR指定父节点。作为运算符，PRIOR和加(+)减(-)运算的优先级相同。condition ... PRIOR expr = expr 或者 ... expr = PRIOR expr
例：
CONNECT BY last_name != 'King' AND PRIOR employee_id = manager_id ...
CONNECT BY PRIOR employee_id = manager_id and PRIOR account_mgr_id = customer_id SYS_CONNECT_BY_PATH
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！
示例
SQL> select empno,mgr,ename,job,level from emp
2 start with empno = 7839
3* connect by prior empno = mgr
EMPNO MGR ENAME JOB LEVEL
---------- ---------- -------- ---------- ----------
7839 KING PRESIDENT 1
7566 7839 JONES MANAGER 2
7788 7566 SCOTT ANALYST 3
7876 7788 ADAMS CLERK 4
7902 7566 FORD ANALYST 3
7369 7902 SMITH CLERK 4
7698 7839 BLAKE MANAGER 2
7499 7698 ALLEN SALESMAN 3
7521 7698 WARD SALESMAN 3
7654 7698 MARTIN SALESMAN 3
7844 7698 TURNER SALESMAN 3
EMPNO MGR ENAME JOB LEVEL
---------- ---------- -------- ---------- ----------
7900 7698 JAMES CLERK 3
7782 7839 CLARK MANAGER 2
7934 7782 MILLER CLERK 3
SQL> select empno,max(sys_connect_by_path(ename,',')) a from emp
2 start with empno=7839
3 connect by prior empno=mgr
4* group by empno
EMPNO A
---------- ------------------------------
7839 ,KING
7844 ,KING,BLAKE,TURNER
7782 ,KING,CLARK
7698 ,KING,BLAKE
7902 ,KING,JONES,FORD
7521 ,KING,BLAKE,WARD
7566 ,KING,JONES
7788 ,KING,JONES,SCOTT
7654 ,KING,BLAKE,MARTIN
7934 ,KING,CLARK,MILLER
7499 ,KING,BLAKE,ALLEN
EMPNO A
---------- ------------------------------
7876 ,KING,JONES,SCOTT,ADAMS
7369 ,KING,JONES,FORD,SMITH
7900 ,KING,BLAKE,JAMES
14 rows selected.
树型结构遍历过程(通过上面的查询来描述)
1).从根节点开始(即where_clause中的条件,如果为非根节点则分根节点作为根节点开始遍历,如上例empno = 7839)
2).遍历根节点(得到empno = 7839记录的相关信息)
3).判断该节点是否存在子节点，如果有则访问最左侧未被访问的子节点，否则下一步。上例中prior_condition为empno = mgr，表示子节点的mgr等于父节点的empno,即下一条返回记录的mgr应当等于前一条记录的empno
4).当节点为叶节点，则访问完毕。
5).返回到该节点的父节点，直至检索完所有数据
=========================================================
格式化层次查询结果，使用2* level - 2个“_”向左填充。修改start with 中的条件，从非根节点开始遍历
SQL> select ename,level,lpad(ename,length(ename)+(level*2-2),'_') as Ename ,job
2 from emp
3 start with ename='JONES'
4* connect by prior empno = mgr
ENAME LEVEL ENAME JOB
--------------- ---------- --------------- ----------
JONES 1 JONES MANAGER
SCOTT 2 __SCOTT ANALYST
ADAMS 3 ____ADAMS CLERK
FORD 2 __FORD ANALYST
SMITH 3 ____SMITH CLERK
也可以从下向上进行遍历
SQL> select ename,level,lpad(ename,length(ename)+(level*2-2),'_') as Ename ,job
2 from emp
3 start with ename='SCOTT'
4* connect by prior mgr = empno
ENAME LEVEL ENAME JOB
--------------- ---------- --------------- ----------
SCOTT 1 SCOTT ANALYST
JONES 2 __JONES MANAGER
KING 3 ____KING PRESIDENT
============================================================
在层次查询中删除节点和分支
通过where子句来过滤SCOTT用户，但SCOTT的下属ADAMS并没有过滤掉
SQL> select ename,level,lpad(ename,length(ename)+(level*2-2),'_') as Ename ,job
2 from emp
3 where ename != 'SCOTT'
4 start with ename='JONES'
5* connect by prior empno = mgr
ENAME LEVEL ENAME JOB
--------------- ---------- --------------- ----------
JONES 1 JONES MANAGER
ADAMS 3 ____ADAMS CLERK
FORD 2 __FORD ANALYST
SMITH 3 ____SMITH CLERK
通过将过滤条件由where 子句的内容移动到connect by prior 子句中过滤掉SCOTT及其下属
SQL> select ename,level,lpad(ename,length(ename)+(level*2-2),'_') as Ename ,job
2 from emp
3 start with ename='JONES'
4* connect by prior empno = mgr and ename != 'SCOTT'
ENAME LEVEL ENAME JOB
--------------- ---------- --------------- ----------
JONES 1 JONES MANAGER
FORD 2 __FORD ANALYST
SMITH 3 ____SMITH CLERK
配合条件查询
SQL> select ename,sal,level,lpad(ename,length(ename)+(level*2-2),'_') as Ename ,job
2 from emp
3 --where sal > 1500
4 start with ename='KING'
5* connect by prior empno = mgr and sal > (select avg(sal) from emp)
ENAME SAL LEVEL ENAME JOB
--------------- ---------- ---------- --------------- ----------
KING 5000 1 KING PRESIDENT
JONES 2975 2 __JONES MANAGER
SCOTT 3000 3 ____SCOTT ANALYST
FORD 3000 3 ____FORD ANALYST
BLAKE 2850 2 __BLAKE MANAGER
CLARK 2450 2 __CLARK MANAGER
参考至：http://blog.csdn.net/robinson_0612/article/details/5616877
http://www.51cto.com/art/200705/47640_1.htm
http://myiteyeverywell-163-com.iteye.com/blog/1471752
http://www.itpub.net/thread-620427-1-1.html
http://blog.chinaunix.net/uid-3634-id-2129988.html
http://blog.sina.com.cn/s/blog_777a9ccb0101740d.html
http://www.cnblogs.com/huanghai223/archive/2010/12/10/1902696.html
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