小知识：Oracle中的层次查询

使用Oracle中的start with .. connect by prior ..语句可以轻松实现。
下面通过scott用户下的emp来做演示，使用自己的一个19c测试环境，结果发现默认并没有scott用户及其测试表，我们需要使用自带脚本添加：

@?/rdbms/admin/utlsampl.sql

发现脚本跑完没有显示报错，但也没有成功创建表，进一步排查发现因为是我们使用的是19c的一个PDB，脚本中的连接库方式默认没有指定，需要修改下。
我们先在tnsnames.ora配置文件中添加这个PDB的配置：

--配置tnsnames.ora
CMDB =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = db19c-scan)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = cmdb)
    )
  )

修正utlsampl.sql脚本中连接库方式，指定配置好的PDB：

...
CONNECT SCOTT/tiger@cmdb 
...

再次执行，scott用户下面熟悉的测试表创建成功。
查询emp表：

SQL> conn scott/tiger@cmdb
Connected.
SQL> show pdbs
SP2-0382: The SHOW PDBS command is not available
SQL> select * from emp;

     EMPNO ENAME      JOB	       MGR HIREDATE	    SAL       COMM     DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
      7369 SMITH      CLERK	      7902 17-DEC-80	    800 		   20
      7499 ALLEN      SALESMAN	      7698 20-FEB-81	   1600        300	   30
      7521 WARD       SALESMAN	      7698 22-FEB-81	   1250        500	   30
      7566 JONES      MANAGER	      7839 02-APR-81	   2975 		   20
      7654 MARTIN     SALESMAN	      7698 28-SEP-81	   1250       1400	   30
      7698 BLAKE      MANAGER	      7839 01-MAY-81	   2850 		   30
      7782 CLARK      MANAGER	      7839 09-JUN-81	   2450 		   10
      7788 SCOTT      ANALYST	      7566 19-APR-87	   3000 		   20
      7839 KING       PRESIDENT 	   17-NOV-81	   5000 		   10
      7844 TURNER     SALESMAN	      7698 08-SEP-81	   1500 	 0	   30
      7876 ADAMS      CLERK	      7788 23-MAY-87	   1100 		   20
      7900 JAMES      CLERK	      7698 03-DEC-81	    950 		   30
      7902 FORD       ANALYST	      7566 03-DEC-81	   3000 		   20
      7934 MILLER     CLERK	      7782 23-JAN-82	   1300 		   10

14 rows selected.

利用层次查询中的伪列level和表达式sys_connect_by_path，查询如下：

select level, ename, job, sys_connect_by_path(ename,'->')
 from emp
 start with mgr is null
 connect by prior empno = mgr
/

查询结果如下：

SQL> col sys_connect_by_path(ename,'->') for a35
SQL> /

     LEVEL ENAME      JOB	SYS_CONNECT_BY_PATH(ENAME,'->')
---------- ---------- --------- -----------------------------------
	 1 KING       PRESIDENT ->KING
	 2 JONES      MANAGER	->KING->JONES
	 3 SCOTT      ANALYST	->KING->JONES->SCOTT
	 4 ADAMS      CLERK	->KING->JONES->SCOTT->ADAMS
	 3 FORD       ANALYST	->KING->JONES->FORD
	 4 SMITH      CLERK	->KING->JONES->FORD->SMITH
	 2 BLAKE      MANAGER	->KING->BLAKE
	 3 ALLEN      SALESMAN	->KING->BLAKE->ALLEN
	 3 WARD       SALESMAN	->KING->BLAKE->WARD
	 3 MARTIN     SALESMAN	->KING->BLAKE->MARTIN
	 3 TURNER     SALESMAN	->KING->BLAKE->TURNER
	 3 JAMES      CLERK	->KING->BLAKE->JAMES
	 2 CLARK      MANAGER	->KING->CLARK
	 3 MILLER     CLERK	->KING->CLARK->MILLER

14 rows selected.

这样就通过start with .. connect by prior ..语句轻松的将这个层次关系查询出来，当然也可以根据需求进一步排序：

SQL> ed
Wrote file afiedt.buf

  1  select level, ename, job, sys_connect_by_path(ename,'->')
  2   from emp
  3   start with mgr is null
  4   connect by prior empno = mgr
  5*  order by 1
SQL> /

     LEVEL ENAME      JOB	SYS_CONNECT_BY_PATH(ENAME,'->')
---------- ---------- --------- -----------------------------------
	 1 KING       PRESIDENT ->KING
	 2 JONES      MANAGER	->KING->JONES
	 2 BLAKE      MANAGER	->KING->BLAKE
	 2 CLARK      MANAGER	->KING->CLARK
	 3 FORD       ANALYST	->KING->JONES->FORD
	 3 WARD       SALESMAN	->KING->BLAKE->WARD
	 3 JAMES      CLERK	->KING->BLAKE->JAMES
	 3 MILLER     CLERK	->KING->CLARK->MILLER
	 3 ALLEN      SALESMAN	->KING->BLAKE->ALLEN
	 3 SCOTT      ANALYST	->KING->JONES->SCOTT
	 3 MARTIN     SALESMAN	->KING->BLAKE->MARTIN
	 3 TURNER     SALESMAN	->KING->BLAKE->TURNER
	 4 ADAMS      CLERK	->KING->JONES->SCOTT->ADAMS
	 4 SMITH      CLERK	->KING->JONES->FORD->SMITH

14 rows selected.

也可以指定关心的员工及其下属关系：

SQL> ed
Wrote file afiedt.buf

  1  select level, ename, job, sys_connect_by_path(ename,'->')
  2   from emp
  3   start with ename = 'SCOTT'
  4   connect by prior empno = mgr
  5*  order by 1
SQL> /

     LEVEL ENAME      JOB	SYS_CONNECT_BY_PATH(ENAME,'->')
---------- ---------- --------- -----------------------------------
	 1 SCOTT      ANALYST	->SCOTT
	 2 ADAMS      CLERK	->SCOTT->ADAMS

第一次看到这类SQL时，总觉得语法很怪，但其实明白其实现的功能后，就会发现这种写法真是既简单又高效。