SQL語句之on子句過濾和where子句過濾區別

一、測試數據：

SQL> select * from dept;
 
DEPTNO DNAME          LOC
------ -------------- -------------
     10 ACCOUNTING     NEW YORK
     20 RESEARCH       DALLAS
     30 SALES          CHICAGO
     40 OPERATIONS     BOSTON
 
SQL> select * from emp;
 
EMPNO ENAME      JOB         MGR HIREDATE          SAL      COMM DEPTNO
----- ---------- --------- ----- ----------- --------- --------- ------
  7369 SMITH      CLERK      7902 1980/12/17     800.00               20
  7499 ALLEN      SALESMAN   7698 1981/2/20     1600.00    300.00     30
  7521 WARD       SALESMAN   7698 1981/2/22     1250.00    500.00     30
  7566 JONES      MANAGER    7839 1981/4/2      2975.00               20
  7654 MARTIN     SALESMAN   7698 1981/9/28     1250.00   1400.00     30
  7698 BLAKE      MANAGER    7839 1981/5/1      2850.00               30
  7782 CLARK      MANAGER    7839 1981/6/9      2450.00               10
  7788 SCOTT      ANALYST    7566 1987/4/19     3000.00               20
  7839 KING       PRESIDENT       1981/11/17    5000.00               10
  7844 TURNER     SALESMAN   7698 1981/9/8      1500.00      0.00     30
  7876 ADAMS      CLERK      7788 1987/5/23     1100.00               20
  7900 JAMES      CLERK      7698 1981/12/3      950.00               30
  7902 FORD       ANALYST    7566 1981/12/3     3000.00               20
  7934 MILLER     CLERK      7782 1982/1/23     1300.00               10
 
14 rows selected

二、左鏈接測試

SQL> select
e.empno, e.ename, e.deptno, d.deptno, d.dname
   3  from scott.emp e left join scott.dept d on e.deptno = d.deptno and e.deptno != 20 and d.deptno != 30;

     EMPNO ENAME                  DEPTNO     DEPTNO DNAME
---------- ------------------------------ ---------- ---------- ------------------------------------------
       7934 MILLER                  10         10 ACCOUNTING
       7839 KING                   10         10 ACCOUNTING
       7782 CLARK                  10         10 ACCOUNTING
       7900 JAMES                  30
       7844 TURNER                  30
       7698 BLAKE                  30
       7654 MARTIN                  30
       7521 WARD                   30
       7499 ALLEN                  30
       7902 FORD                   20
       7876 ADAMS                  20
       7788 SCOTT                  20
       7566 JONES                  20
       7369 SMITH                  20

14 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 3387915970

---------------------------------------------------------------------------
| Id  | Operation       | Name | Rows  | Bytes | Cost (%CPU)| Time      |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |    14 |   364 |    7  (15)| 00:00:01 |
|*  1 |  HASH JOIN OUTER   |      |    14 |   364 |    7  (15)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| EMP  |    14 |   182 |    3   (0)| 00:00:01 |
|*  3 |   TABLE ACCESS FULL| DEPT |    3 |    39 |    3   (0)| 00:00:01 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("E"."DEPTNO"="D"."DEPTNO"(+))
        filter("E"."DEPTNO"<>CASE  WHEN ("D"."DEPTNO"(+) IS NOT NULL)
           THEN 20 ELSE 20 END )
    3 - filter("D"."DEPTNO"(+)<>30)

Statistics
----------------------------------------------------------
       0  recursive calls
       0  db block gets
      13  consistent gets
       0  physical reads
       0  redo size
        1082  bytes sent via SQL*Net to client
     524  bytes received via SQL*Net from client
       2  SQL*Net roundtrips to/from client
       0  sorts (memory)
       0  sorts (disk)
      14  rows processed

    
-- 結論：left join 僅有on子句
-- 過濾條件對左表無效；

-- on子句過濾條件僅對右表生效；
-- 對於右表，過濾條件在鏈接以前生效（即先過濾，後鏈接）；
-- 對於左表，過濾條件在鏈接以後生效（即先鏈接，後過濾）。
     
SQL> select
e.empno, e.ename, e.deptno, d.deptno, d.dname
   3  from scott.emp e left join scott.dept d on e.deptno = d.deptno where e.deptno != 20 and d.deptno != 30;

     EMPNO ENAME                  DEPTNO     DEPTNO DNAME
---------- ------------------------------ ---------- ---------- ------------------------------------------
       7782 CLARK                  10         10 ACCOUNTING
       7934 MILLER                  10         10 ACCOUNTING
       7839 KING                   10         10 ACCOUNTING

Execution Plan
----------------------------------------------------------
Plan hash value: 844388907

----------------------------------------------------------------------------------------
| Id  | Operation             | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |           |     9 |   234 |     6    (17)| 00:00:01 |
|   1 |  MERGE JOIN             |           |     9 |   234 |     6    (17)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| DEPT    |     3 |    39 |     2     (0)| 00:00:01 |
|*  3 |    INDEX FULL SCAN         | PK_DEPT |     3 |       |     1     (0)| 00:00:01 |
|*  4 |   SORT JOIN             |           |     9 |   117 |     4    (25)| 00:00:01 |
|*  5 |    TABLE ACCESS FULL         | EMP     |     9 |   117 |     3     (0)| 00:00:01 |
----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - filter("D"."DEPTNO"<>30 AND "D"."DEPTNO"<>20)
    4 - access("E"."DEPTNO"="D"."DEPTNO")
        filter("E"."DEPTNO"="D"."DEPTNO")
    5 - filter("E"."DEPTNO"<>20 AND "E"."DEPTNO"<>30)

Statistics
----------------------------------------------------------
       0  recursive calls
       0  db block gets
      10  consistent gets
       0  physical reads
       0  redo size
     916  bytes sent via SQL*Net to client
     524  bytes received via SQL*Net from client
       2  SQL*Net roundtrips to/from client
       1  sorts (memory)
       0  sorts (disk)
       3  rows processed

-- 結論：where子句過濾
-- 過濾條件對於左右表都有效；
-- 對於右表，過濾條件在鏈接以前生效（即先過濾，後鏈接）；
-- 對於左表，過濾條件在鏈接以前生效（即先過濾，後鏈接）；
-- 對於左鏈接，右表過濾字段出如今鏈接條件中，左連解會變成內鏈接。

三、如下SQL自行測試，可驗證以上結論：

select
e.empno, e.ename, e.deptno, d.deptno, d.dname
from scott.dept d join scott.emp e on e.deptno = d.deptno where e.deptno != 20 and d.deptno != 30;
  
select
e.empno, e.ename, e.deptno, d.deptno, d.dname
from scott.dept d join scott.emp e on e.deptno = d.deptno where d.deptno != 30;
  
select
e.empno, e.ename, e.deptno, d.deptno, d.dname
from scott.dept d join scott.emp e on e.deptno = d.deptno where e.deptno != 20;

select e.empno, e.ename, e.deptno, d.deptno, d.dname from scott.dept d left join scott.emp e on e.deptno = d.deptno where e.deptno != 20 and d.deptno != 30;   select e.empno, e.ename, e.deptno, d.deptno, d.dname from scott.dept d left join scott.emp e on e.deptno = d.deptno where d.deptno != 30;   select e.empno, e.ename, e.deptno, d.deptno, d.dname from scott.dept d left join scott.emp e on e.deptno = d.deptno where e.deptno != 20;