[每日一题] OCP1z0-047 :2013-08-11描述层次查询（hierarchical query）
正确答案：BD
A错误,树的遍历可以从上至下，或从下至上
B正确
C正确,可以删除某个某个遍历的分支
例: 删除scott的分支
SQL> select empno,ename,level,lpad('',2*level-1)||sys_connect_by_path(ename,'/') path from emp
2 start with empno=7566
3 connect by priorempno=mgr and ename!='SCOTT';
D错误
SQL> select empno,ename,level,lpad('',2*level-1)||sys_connect_by_path(ename,'/') path from emp
2 where ename!='SCOTT'
3 start with empno=7566
4 connect by priorempno=mgr;
可以使用条件限制输出。
正确答案BC
EMPNO ENAME LEVEL PATH
---------- ---------- ----------------------------------------
7566 JONES 1 /JONES
7876 ADAMS 3 /JONES/SCOTT/ADAMS
7902 FORD 2 /JONES/FORD
7369 SMITH 3 /JONES/FORD/SMITH
EMPNO ENAME LEVEL PATH
---------- ---------- ----------------------------------------
7566 JONES 1 /JONES
7902 FORD 2 /JONES/FORD
7369 SMITH 3 /JONES/FORD/SMITH
层次查询知识补充：
[html] gyj@MYDB> create table test(id number,name varchar2(10),fid number); Table created. gyj@MYDB> insert into test values(1,'A',2); 1 row created. gyj@MYDB> insert into test values(2,'B',3); 1 row created. gyj@MYDB> insert into test values(3,'C',4); 1 row created. gyj@MYDB> insert into test values(4,'D',null); 1 row created. gyj@MYDB> commit; Commit complete.
正向查找，对于每个遍历，只查找第一行记录
[html] gyj@MYDB> select distinct first_value(path) over(partition by id order by lev desc) from ( 2 select connect_by_root id id,level lev, sys_connect_by_path(name,' ') path 3 from test 4 start with id in (select id from test) 5 connect by id=prior fid); FIRST_VALUE(PATH)OVER(PARTITIONBYIDORDERBYLEVDESC) --------------------------------------------------------------------------------------- A B C D B C D C D D
正向查找，用翻转函数
[html] gyj@MYDB> select reverse(sys_connect_by_path(name,' ')) 2 from test 3 start with fid is null 4 connect by fid= prior id 5 order by level desc; REVERSE(SYS_CONNECT_BY_PATH(NAME,'')) --------------------------------------------------------------------------------------- A B C D B C D C D D
反向查找，最后只找叶子节点
[html] gyj@MYDB> SELECT SYS_CONNECT_BY_PATH(NAME,' ') 2 FROM TEST 3 WHERE CONNECT_BY_ISLEAF=1 4 START WITH ID IS NOT NULL --× id is not null, ×÷ ù ò é 5 CONNECT BY ID=PRIOR FID; SYS_CONNECT_BY_PATH(NAME,'') --------------------------------------------------------------------------------------- A B C D B C D C D D
使用10g reverse函数
[html] gyj@MYDB> WITH TEMP AS 2 ( 3 SELECT 1 ID,'A' NAME,2 PARENT FROM DUAL 4 UNION 5 SELECT 2 ID,'B' NAME,3 PARENT FROM DUAL 6 UNION 7 SELECT 3 ID,'C' NAME,4 PARENT FROM DUAL 8 UNION 9 SELECT 4 ID,'D' NAME,NULL PARENT FROM DUAL 10 ) 11 SELECT REVERSE(NAME) FROM 12 ( 13 SELECT SYS_CONNECT_BY_PATH(NAME,' ') NAME,LENGTH(SYS_CONNECT_BY_PATH(NAME,' ')) RN FROM TEMP CONNECT BY PARENT = PRIOR ID START WITH PARENT IS NULL 14 ) ORDER BY RN DESC; REVERSE(NAME) --------------------------------------------------------------------------------------- A B C D B C D C D D
connect by和where，where是对最后的结果的过滤，不影响connect by出来的层次关系：也就是节点的level,所属的父节点，根等不变,不影响最后的结果。
[html]
gyj@MYDB> select a.t,b.t
2 from (select rownum n, subs