我們經常使用 join相關語句做關聯查詢，那麼在join連線方式後邊，on 結合and 和 where結果會發生什麼變化呢？

在使用 join on 時 注意 and where 區別和如何使用

join on and
join on and 方式 類似於 on 條件1 and on 條件2，都是 基於join 關聯兩個表結果 ，取出關聯後資料。 舉例如下
select t2.object_id t2_id from t1 right join t2 on t1.object_id=t2.object_id and t1.object_id=1989;
--輸出結果太多，省略，看 下邊access部分
92937 rows selected.
Elapsed: 00:00:05.33
Execution Plan

Plan hash value: 2539735012

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

| 0 | SELECT STATEMENT | | 102K| 2609K| 372 (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 右連線 t2的 object_id，以t2輸出為主，也就是輸出t2所有內容
2 - access("T1"."OBJECT_ID"(+)=1989) --同上，輸出1989所有頁遊內容（這個條件在此可以忽略）

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連線之後對結果再進行篩選(為達到執行效率最有，是先進性where條件篩選，再join關聯），舉例如下
SQL> select t2.object_id t2_id from t1 www.sangpi.comright join t2 on t1.object_id=t2.object_id where t1.object_id=1989;

T2_ID

1989

Elapsed: 00:00:00.06
Execution Plan

Plan hash value: 2511910206

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

| 0 | SELECT STATEMENT | | 1 | 26 | 2 (0)| 00:00:01 |
| 1 | MERGE JOIN CARTESIAN| | 1 | 26 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | T2_IDX | 1 | 13 | 1 (0)| 00:00:01 |
| 3 | BUFFER SORT | | 1 | 13 | 1 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | T1_IDX | 1 | 13 | 1 (0)| 00:00:01 |

Predicate Information (identified by operation id):

2 - access("T2"."OBJECT_ID"=1989)
4 - access("T1"."OBJECT_ID"=1989)
Note

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

Statistics

 21  recursive calls
  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