[SQL学习] [Oracle] 层次化查询 (start with connect by)

层次化查询 (START BY ... CONNECT BY PRIOR)

SELECT [LEVEL] ,column,expression,...
FROM table_name
[WHERE where_clause]
[[START WITH start_condition] [CONNECT BY PRIOR prior_condition]];

LEVEL: 为伪列, 用于表示树的层次
start_condition: 层次化查询的起始条件
prior_condition: 定义父节点和子节点之间的关系 (父主键 = 子外键)


1 使用 start with ... connect by prior 从根节点开始遍历

select empno,mgr,ename,job
from emp
start with empno = 7839
connect by prior empno = mgr
/

    EMPNO        MGR ENAME      JOB
---------- ---------- ---------- ---------
     7839            KING       PRESIDENT
     7566       7839 JONES      MANAGER
     7788       7566 SCOTT      ANALYST
     7876       7788 ADAMS      CLERK
     7902       7566 FORD       ANALYST
     7369       7902 SMITH      CLERK
     7698       7839 BLAKE      MANAGER
     7499       7698 ALLEN      SALESMAN
     7521       7698 WARD       SALESMAN
     7654       7698 MARTIN     SALESMAN
     7844       7698 TURNER     SALESMAN
     7900       7698 JAMES      CLERK
     7782       7839 CLARK      MANAGER
     7934       7782 MILLER     CLERK

14 rows selected.


2 伪列 level 的使用

注意 connect by prior empno = mgr 的理解
prior 表示前一条记录,即下一条返回记录的 mgr 应当等于前一条记录的 empno
select level,empno,mgr,ename,job
from emp
start with ename = 'KING'
connect by prior empno = mgr
order by level;

    LEVEL      EMPNO        MGR ENAME      JOB

---------- ---------- ---------- ---------- ---------
        1       7839            KING       PRESIDENT
        2       7566       7839 JONES      MANAGER
        2       7698       7839 BLAKE      MANAGER
        2       7782       7839 CLARK      MANAGER
        3       7902       7566 FORD       ANALYST
        3       7521       7698 WARD       SALESMAN
        3       7900       7698 JAMES      CLERK
        3       7934       7782 MILLER     CLERK
        3       7499       7698 ALLEN      SALESMAN
        3       7788       7566 SCOTT      ANALYST
        3       7654       7698 MARTIN     SALESMAN
        3       7844       7698 TURNER     SALESMAN
        4       7876       7788 ADAMS      CLERK
        4       7369       7902 SMITH      CLERK


3 获得层次数

select count(distinct level) "Level"
from emp
start with ename = 'KING'
connect by prior empno = mgr
/
    Level

----------

        4


4 格式化层次查询结果
col Ename for a30
select level,lpad(' ',2 * level - 1) || ename as "Ename",job
from emp
start with ename = 'KING'
connect by prior empno = mgr
/

    LEVEL Ename                          JOB

---------- ------------------------------ ---------
        1  KING                          PRESIDENT
        2    JONES                       MANAGER
        3      SCOTT                     ANALYST
        4        ADAMS                   CLERK
        3      FORD                      ANALYST
        4        SMITH                   CLERK
        2    BLAKE                       MANAGER
        3      ALLEN                     SALESMAN
        3      WARD                      SALESMAN
        3      MARTIN                    SALESMAN
        3      TURNER                    SALESMAN
        3      JAMES                     CLERK
        2    CLARK                       MANAGER
        3      MILLER                    CLERK

14 rows selected.


5 从非根节点开始遍历(只需修改start with 中的条件即可)

select level,lpad(' ',2 * level - 1) || ename as "Ename",job
from emp
start with ename = 'SCOTT'
connect by prior empno = mgr
/

    LEVEL Ename                          JOB
---------- ------------------------------ ---------
        1  SCOTT                         ANALYST
        2    ADAMS                       CLERK


6 从下向上遍历(交换connect by prior中的条件即可,使用mgr = empno)

注意connect by prior mgr = empno 的理解
prior表示前一条记录,即下一条返回记录的empno应当等于前一条记录的mgr

select level,lpad(' ',2 * level - 1) || ename as "Ename",job
from emp
start with ename = 'SCOTT'
connect by prior mgr = empno
/


    LEVEL Ename                          JOB
---------- ------------------------------ ---------
        1  SCOTT                         ANALYST
        2    JONES                       MANAGER
        3      KING                      PRESIDENT


7 从下向上遍历(也可以将prior置于等号右边,得到相同的结果)
select level,
lpad(' ',2 * level - 1) || ename as "Ename",job
from emp
start with ename = 'SCOTT'
connect by empno = prior mgr
/


    LEVEL Ename                          JOB
---------- ------------------------------ ---------
        1  SCOTT                         ANALYST
        2    JONES                       MANAGER
        3      KING                      PRESIDENT

8 从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

select level,lpad(' ',2 * level - 1) || ename as "Ename",job
from emp
where ename != 'SCOTT'
start with empno = 7839
connect by prior empno = mgr
/

     LEVEL Ename                JOB
---------- -------------------- ---------
        1  KING                PRESIDENT
        2    JONES             MANAGER
        4        ADAMS         CLERK
        3      FORD            ANALYST
        4        SMITH         CLERK
        2    BLAKE             MANAGER
        3      ALLEN           SALESMAN
        3      WARD            SALESMAN
        3      MARTIN          SALESMAN
        3      TURNER          SALESMAN
        3      JAMES           CLERK
        2    CLARK             MANAGER
        3      MILLER          CLERK

13 rows selected.


9 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

select level,lpad(' ',2 * level - 1) || ename as "Ename",job
from emp
start with empno = 7839
connect by prior empno = mgr and ename != 'SCOTT'
/

   LEVEL Ename                JOB
--------- -------------------- ---------
       1  KING                PRESIDENT
       2    JONES             MANAGER
       3      FORD            ANALYST
       4        SMITH         CLERK
       2    BLAKE             MANAGER
       3      ALLEN           SALESMAN
       3      WARD            SALESMAN
       3      MARTIN          SALESMAN
       3      TURNER          SALESMAN
       3      JAMES           CLERK
       2    CLARK             MANAGER
       3      MILLER          CLERK

2 rows selected.

10 在层次化查询中增加过滤条件或使用子查询

select level,lpad(' ',2 * level - 1) || ename as "Ename",job
from emp
where sal > 2500
start with empno = 7839
connect by prior empno = mgr
/

   LEVEL Ename                JOB
--------- -------------------- ---------
       1  KING                PRESIDENT
       2    JONES             MANAGER
       3      SCOTT           ANALYST
       3      FORD            ANALYST
       2    BLAKE             MANAGER

select level,lpad(' ',2 * level - 1) || ename as "Ename",job
from emp
where sal > (select avg(sal) from emp)
start with empno = 7839
connect by prior empno = mgr
/

   LEVEL Ename                JOB
--------- -------------------- ---------
       1  KING                PRESIDENT
       2    JONES             MANAGER
       3      SCOTT           ANALYST
       3      FORD            ANALYST
       2    BLAKE             MANAGER
       2    CLARK             MANAGER

6 rows selected.