quote : http://xo-tobacoo.iteye.com/blog/182791
總結下。使用merge比傳統的先判斷再選擇插入或更新快很多。
1)主要功能
提供有條件地更新和插入數據到數據庫表中
如果該行存在,執行一個UPDATE操作,如果是一個新行,執行INSERT操作
— 避免了分開更新
— 提高性能並易於使用
— 在數據倉庫應用中十分有用
2)MERGE語句的語法如下:
MERGE [hint] INTO [schema .] table [t_alias] USING [schema .]
{ table | view | subquery } [t_alias] ON ( condition )
WHEN MATCHED THEN merge_update_clause
WHEN NOT MATCHED THEN merge_insert_clause;
還是看例子就知道怎麼回事:
MERGE INTO copy_emp c
USING employees e
ON (c.employee_id=e.employee_id)
WHEN MATCHED THEN
UPDATE SET
c.first_name=e.first_name,
c.last_name=e.last_name,
c.department_id=e.department_id
WHEN NOT MATCHED THEN
INSERT VALUES(e.employee_id,e.first_name,e.last_name,
e.email,e.phone_number,e.hire_date,e.job_id,
e.salary,e.commission_pct,e.manager_id,
e.departmetn_id);
MERGE INTO copy_emp c
USING employees e
ON (c.employee_id=e.employee_id)
WHEN MATCHED THEN
UPDATE SET
c.first_name=e.first_name,
c.last_name=e.last_name,
c.department_id=e.department_id
WHEN NOT MATCHED THEN
INSERT VALUES(e.employee_id,e.first_name,e.last_name,
e.email,e.phone_number,e.hire_date,e.job_id,
e.salary,e.commission_pct,e.manager_id,
e.departmetn_id);
3)使用merge的注意事項:
創建測試表:
CREATE TABLE MM (ID NUMBER, NAME VARCHAR2(20));
CREATE TABLE MN (ID NUMBER, NAME VARCHAR2(20));
插入數據
INSERT INTO MM VALUES (1, 'A');
INSERT INTO MN VALUES (1, 'B');
執行:
MERGE INTO MN A
USING MM B
ON(A.ID=B.ID)
WHEN MATCHED THEN
UPDATE SET A.ID = B.ID
WHEN NOT MATCHED THEN
INSERT VALUES(B.ID, B.NAME);
ON(A.ID=B.ID)
報錯,原因是on子句的使用的字段不能夠用於update,即Oracle不允許更新用於連接的列
修改:
MERGE INTO MN A
USING MM B
ON(A.ID=B.ID)
WHEN MATCHED THEN
UPDATE SET A.NAME = B.NAME
WHEN NOT MATCHED THEN
INSERT VALUES(B.ID, B.NAME);
ON(A.ID=B.ID)
再插入:INSERT INTO MM VALUES (1, 'C');
再執行:
MERGE INTO MN A
USING MM B
ON(A.ID=B.ID)
WHEN MATCHED THEN
UPDATE SET A.NAME = B.NAME
WHEN NOT MATCHED THEN
INSERT VALUES(B.ID, B.NAME);
ON(A.ID=B.ID)
報錯,原因無法在源表中獲得一組穩定的行
4)更新同一張表的數據。需要注意下細節,因爲可能涉及到using的數據集爲null,所以要使用count()函數。
MERGE INTO mn a
USING (select count(*) co from mn where mn.ID=4) b
ON (b.co<>0)--這裏使用了count和<>,注意下,想下爲什麼!
WHEN MATCHED THEN
UPDATE
SET a.NAME = 'E'
where a.ID=4
WHEN NOT MATCHED THEN
INSERT
VALUES (4, 'E');