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这篇文章主要介绍Oracle的表如何连接，文中介绍的非常详细，具有一定的参考价值，感兴趣的小伙伴们一定要看完！
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Oracle表连接方法有四种：
● 排序合并连接（Sort Merge Join）
● 嵌套循环连接（Nested Loops Join）
● 哈希连接（Hash Join）
● 笛卡尔积（Cartesian Product）
如果where条件有exists、in或=any操作符+子查询，则Oracle会将其处理为半连接，执行计划中对应的关键字为SEMI。
SQL> select department_id,department_name from departments d where exists (select 1 from employees e where d.department_id = e.department_id and e.salary > 2500);
11 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2188966913
--------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 230 | 6 (17)| 00:00:01 |
| 1 | MERGE JOIN SEMI | | 10 | 230 | 6 (17)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPARTMENTS | 27 | 432 | 2 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | DEPT_ID_PK | 27 | | 1 (0)| 00:00:01 |
|* 4 | SORT UNIQUE | | 105 | 735 | 4 (25)| 00:00:01 |
|* 5 | TABLE ACCESS FULL | EMPLOYEES | 105 | 735 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------
SQL> select department_id,department_name from departments d where department_id in (select department_id from employees e where d.department_id = e.department_id and e.salary > 2500);
11 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2188966913
--------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 230 | 6 (17)| 00:00:01 |
| 1 | MERGE JOIN SEMI | | 10 | 230 | 6 (17)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPARTMENTS | 27 | 432 | 2 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | DEPT_ID_PK | 27 | | 1 (0)| 00:00:01 |
|* 4 | SORT UNIQUE | | 105 | 735 | 4 (25)| 00:00:01 |
|* 5 | TABLE ACCESS FULL | EMPLOYEES | 105 | 735 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------
SQL> select department_id,department_name from departments d where department_id=any(select department_id from employees e where d.department_id = e.department_id and e.salary > 2500);
11 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2188966913
--------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 230 | 6 (17)| 00:00:01 |
| 1 | MERGE JOIN SEMI | | 10 | 230 | 6 (17)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPARTMENTS | 27 | 432 | 2 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | DEPT_ID_PK | 27 | | 1 (0)| 00:00:01 |
|* 4 | SORT UNIQUE | | 105 | 735 | 4 (25)| 00:00:01 |
|* 5 | TABLE ACCESS FULL | EMPLOYEES | 105 | 735 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------------------------
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