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关于前端:and-where-应区别如何使用

jiezi

咱们常常应用 join 相干语句做关联查问，那么在 join 连贯形式后边，on 联合 and 和 where 后果会产生什么变动呢？

在应用 join on 时 留神 and where 区别和如何应用

join on and

join on and 形式 相似于 on 条件 1 and on 条件 2，都是 基于 join 关联两个表后果，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;
– 输入后果太多，省略，看 下边 access 局部
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 右连贯 t2 的 object_id，以 t2 输入为主，也就是输入 t2 所有内容
2 – access(“T1”.”OBJECT_ID”http://www.coubai.com(+)=1989) – 同上，输入 1989 所有内容（这个条件在此能够疏忽）
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 连贯之后对后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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