DB2 Merge 語句的作用非常強大,它可以將一個表中的數據合併到另一個表中,在合併的同時可以進行插入、刪除、更新等操作。我們還是先來看個簡單的例子吧,假設你定義了一個僱員表(employe),一個經理表(manager),如下所示:
---僱員表(EMPLOYE)
CREATE TABLE EMPLOYE (
EMPLOYEID INTEGER NOT NULL,---員工號
NAME VARCHAR(20) NOT NULL,---姓名
SALARY DOUBLE---薪水
);
INSERT INTO EMPLOYE (EMPLOYEID,NAME,SALARY) VALUES
(1,'張三',1000),
(2,'李四',2000),
(3,'王五',3000),
(4,'趙六',4000),
(5,'高七',5000);
--經理表(MANAGER)
CREATE TABLE MANAGER (
EMPLOYEID INTEGER NOT NULL,---經理號
NAME VARCHAR(20) NOT NULL,---姓名
SALARY DOUBLE---薪水
);
INSERT INTO MANAGER (MANAGERID,NAME,SALARY) VALUES
(3,'王五',5000),
(4,'趙六',6000);
經過一段時間,你發現這樣的數據模型,或者說表結構設計簡直就是一大敗筆,經理和僱員都是員工嘛,爲什麼要設計兩個表呢?發現錯誤後就需要改正,所以你決定,刪除經理表(MANAGER)表,將MANAGER 表中的數據合併到EMPLOYE 表中,仔細分析發現,王五在兩個表中都存在(可能是乾的好升官了),而劉八在EMPLOYE 表中並不存在,現在,我們要求把EMPLOYE 表中不存在的MANAGER都插入到EMPLOYE 表中,存在的更新薪水。該怎麼辦呢?這個問題並不難,通常,我們可以分兩步,如下所示:
--更新存在的
UPDATE EMPLOYE AS EM SET SALARY=(SELECT SALARY FROM MANAGER WHERE MANAGERID=EM.EMPLOYEID)
WHERE EMPLOYEID IN (
SELECT MANAGERID FROM MANAGER
);
---插入不存在的
INSERT INTO EMPLOYE (EMPLOYEID,NAME,SALARY)
SELECT MANAGERID,NAME,SALARY FROM MANAGER WHERE MANAGERID NOT IN (
SELECT EMPLOYEID FROM EMPLOYE
);
上面的處理是可以的,但是我們還可以有更簡單的方法,就是用Merge語句,如下所示:
MERGE INTO EMPLOYE AS EM
USING MANAGER AS MA
ON EM.EMPLOYEID=MA.MANAGERID
WHEN MATCHED THEN UPDATE SET EM.SALARY=MA.SALARY
WHEN NOT MATCHED THEN INSERT VALUES (MA.MANAGERID,MA.NAME,MA.SALARY);
在上面的處理中,我們用經理表(MANAGER)的薪水更新了僱員表(EMPLOYE)的薪水,假設現在要求,如果經理表(MANAGER)的薪水>僱員表(EMPLOYE)的薪水的時候更新,否則不更新,怎麼辦呢?如下:
-1
MERGE INTO EMPLOYE AS EM
USING MANAGER AS MA
ON EM.EMPLOYEID=MA.MANAGERID
WHEN MATCHED AND EM.SALARY<MA.SALARY THEN UPDATE SET EM.SALARY=MA.SALARY
WHEN NOT MATCHED THEN INSERT VALUES (MA.MANAGERID,MA.NAME,MA.SALARY);
-2
USING MANAGER AS MA
ON EM.EMPLOYEID=MA.MANAGERID
WHEN MATCHED AND EM.SALARY<MA.SALARY THEN UPDATE SET EM.SALARY=MA.SALARY
WHEN NOT MATCHED THEN INSERT VALUES (MA.MANAGERID,MA.NAME,MA.SALARY)
else ignore
不仔細的朋友可能沒有看出上面兩條語句的區別,哈哈,請仔細對比一下這兩條語句。上面的語句中多了ELSE IGNORE語句,它的意思正如它英文的意思,其它情況忽略不處理。
如果你認爲理論上應該不存在EM.SALARY>MA.SALARY的數據,如果有,說明有問題,你想拋個異常,怎麼辦?如下:
MERGE INTO EMPLOYE AS EM
USING MANAGER AS MA
ON EM.EMPLOYEID=MA.MANAGERID
WHEN MATCHED AND EM.SALARY<MA.SALARY THEN UPDATE SET EM.SALARY=MA.SALARY
WHEN MATCHED AND EM.SALARY>MA.SALARY THEN SIGNAL SQLSTATE '70001' SET MESSAGE_TEXT = 'EM.SALARY>MA.SALARY'
WHEN NOT MATCHED THEN INSERT VALUES (MA.MANAGERID,MA.NAME,MA.SALARY)
ELSE IGNORE;
對於EM.SALARY>MA.SALARY的情況,如果你不想拋異常,而是刪除EMPLOYE中的數據,怎麼辦?如下:
MERGE INTO EMPLOYE AS EM
USING MANAGER AS MA
ON EM.EMPLOYEID=MA.MANAGERID
WHEN MATCHED AND EM.SALARY<MA.SALARY THEN UPDATE SET EM.SALARY=MA.SALARY
WHEN MATCHED AND EM.SALARY>MA.SALARY THEN DELETE
WHEN NOT MATCHED THEN INSERT VALUES (MA.MANAGERID,MA.NAME,MA.SALARY)
ELSE IGNORE;