近期在网上看到一个这样的SQL需求，需求源自scott.emp表中的数据：
create or replace view v1 as select * from scott.emp;
create or replace view v2 as select * from scott.emp;
SQL> select * from v1;
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.
SQL的需求是，得到员工编号、员工姓名、工作类别、主管编号、主管姓名信息。网上的网友是这么实现的：
SQL> select a.empno,a.ename,a.job,a.mgr as v1_mgr,b.empno as v2_mgr,b.ename from v1 a left join v2 b on a.MGR=b.empno order by 1;
EMPNO ENAME JOB V1_MGR V2_MGR ENAME
---------- ---------- --------- ---------- ---------- ----------
7369 SMITH CLERK 7902 7902 FORD
7499 ALLEN SALESMAN 7698 7698 BLAKE
7521 WARD SALESMAN 7698 7698 BLAKE
7566 JONES MANAGER 7839 7839 KING
7654 MARTIN SALESMAN 7698 7698 BLAKE
7698 BLAKE MANAGER 7839 7839 KING
7782 CLARK MANAGER 7839 7839 KING
7788 SCOTT ANALYST 7566 7566 JONES
7839 KING PRESIDENT
7844 TURNER SALESMAN 7698 7698 BLAKE
7876 ADAMS CLERK 7788 7788 SCOTT
7900 JAMES CLERK 7698 7698 BLAKE
7902 FORD ANALYST 7566 7566 JONES
7934 MILLER CLERK 7782 7782 CLARK
14 rows selected.
Plan hash value: 2322654302
--------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers |
--------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 7 (100)| | 14 |00:00:00.01 | 12 |
| 1 | SORT ORDER BY | | 1 | 14 | 7 (15)| 00:00:01 | 14 |00:00:00.01 | 12 |
|* 2 | HASH JOIN OUTER | | 1 | 14 | 6 (0)| 00:00:01 | 14 |00:00:00.01 | 12 |
| 3 | TABLE ACCESS FULL| EMP | 1 | 14 | 3 (0)| 00:00:01 | 14 |00:00:00.01 | 6 |
| 4 | TABLE ACCESS FULL| EMP | 1 | 14 | 3 (0)| 00:00:01 | 14 |00:00:00.01 | 6 |
--------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("MGR"="EMPNO")
此方法最终实现了目标需求，利用了自关联的方式，也就是emp表需要扫描2次，小表是OK的，emp表中主管与员工是存在层级关系的，我们能否利用上层级查询消除自关联？
SQL> set line 1000 pagesize 200
col new_name for a20
col new_mgr for a20
col mgr_name for a60
select empno,ename,job,mgr,mgr_name from (
select empno,ename,job,mgr,path,regexp_substr(path,'[^,]+',1,decode((lv-1),0,null,(lv-1))) as mgr_name from (
SELECT empno,
ename,
job,
mgr,
LEVEL lv,
sys_connect_by_path(ename,',') AS path
FROM emp connect by prior empno=mgr start with mgr is null));SQL> SQL> SQL> SQL> 2 3 4 5 6 7 8 9
EMPNO ENAME JOB MGR MGR_NAME
---------- ---------- --------- ---------- ------------------------------------------------------------
7839 KING PRESIDENT
7566 JONES MANAGER 7839 KING
7788 SCOTT ANALYST 7566 JONES
7876 ADAMS CLERK 7788 SCOTT
7902 FORD ANALYST 7566 JONES
7369 SMITH CLERK 7902 FORD
7698 BLAKE MANAGER 7839 KING
7499 ALLEN SALESMAN 7698 BLAKE
7521 WARD SALESMAN 7698 BLAKE
7654 MARTIN SALESMAN 7698 BLAKE
7844 TURNER SALESMAN 7698 BLAKE
7900 JAMES CLERK 7698 BLAKE
7782 CLARK MANAGER 7839 KING
7934 MILLER CLERK 7782 CLARK
14 rows selected.
Plan hash value: 422387769
-----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers |
-----------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 3 (100)| | 14 |00:00:00.01 | 2 |
| 1 | VIEW | | 1 | 3 | 3 (34)| 00:00:01 | 14 |00:00:00.01 | 2 |
|* 2 | CONNECT BY NO FILTERING WITH START-WITH| | 1 | | | | 14 |00:00:00.01 | 2 |
| 3 | TABLE ACCESS FULL | EMP | 1 | 14 | 2 (0)| 00:00:01 | 14 |00:00:00.01 | 2 |
-----------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("MGR"=PRIOR NULL)
filter("MGR" IS NULL)
利用sys_connect_by_path与regexp_substr利用层级关系截取，消除了自关联，并且emp表只访问了1次。