這篇文章主要介紹Oracle的表如何連接，文中介紹的非常詳細(xì)，具有一定的參考價(jià)值，感興趣的小伙伴們一定要看完！
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Oracle表連接方法有四種：
● 排序合并連接（Sort Merge Join）
● 嵌套循環(huán)連接（Nested Loops Join）
● 哈希連接（Hash Join）
● 笛卡爾積（Cartesian Product）
如果where條件有exists、in或=any操作符+子查詢，則Oracle會(huì)將其處理為半連接，執(zhí)行計(jì)劃中對(duì)應(yīng)的關(guān)鍵字為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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