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and where 应区别如何使用?

开发技术 开发技术 3小时前 1次浏览

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


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