and where 应区别如何使用？

Posted 2021-09-03 diuxie

我们经常使用 join相关语句做关联查询，那么在join连接方式后边，on 结合and 和 where结果会发生什么变化呢？

在使用 join on 时 注意 and where 区别和如何使用

join on and

join on and 方式 类似于 on 条件1 and on 条件2，都是 基于join 关联两个表结果 ，取出关联后数据。 举例如下

select t2.object_id t2_id from t1 right join t2 on t1.object_id=t2.object_id and t1.object_id=1989;
--输出结果太多，省略，看 下边access部分
92937 rows selected.
Elapsed: 00:00:05.33
Execution Plan
(1)| 00:00:05 |
|* 1 | HASH JOIN RIGHT OUTER| | 102K| 2609K| 372 (1)| 00:00:05 |
|* 2 | INDEX RANGE SCAN | T1_IDX | 1 | 13 | 1 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL | T2 | 102K| 1304K| 371 (1)| 00:00:05 |

Predicate Information (identified by operation id):

1 - access("T1"."OBJECT_ID"(+)="T2"."OBJECT_ID") -- t1 右连接 t2的 object_id，以t2输出为主，也就是输出t2所有内容
2 - access("T1"."OBJECT_ID"http://www.coubai.com(+)=1989) --同上，输出1989所有内容（这个条件在此可以忽略）
Note

• dynamic sampling used for this statement (level=2)

Statistics

 16  recursive calls
  0  db block gets
   7580  consistent gets
  1  physical reads
  0  redo size
1699670  bytes sent via SQL*Net to client
  68668  bytes received via SQL*Net from client
   6197  SQL*Net roundtrips to/from client
  0  sorts (memory)
  0  sorts (disk)
  92937  rows processed1

join on where

join on where ，是 join on连接之后对结果再进页游进行筛选(为达到执行效率最有，是先进性where条件筛选，再join关联），举例如下

SQL> select t2.object_id t2_id from t1 right join t2 on t1.object_id=t2.object_id where t1.object_id=1989;

 T2_ID

  1989

Elapsed: 00:00:00.06
Execution Plan
2 - access("T2"."OBJECT_ID"=1989)
4 - access("T1"."OBJECT_ID"=1989)
Note

• dynamic sampling used for this statement (level=2)

Statistics

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