Oracle Merge 使用介绍

Oracle9i引入了MERGE命令,你能够在一个SQL语句中对一个表同时执行inserts和updates操作. MERGE命令从一个或多个数据源中选择行来updating或inserting到一个或多个表.在Oracle10g中MERGE有如下一些改进：
1、UPDATE或INSERT子句是可选的
2、UPDATE和INSERT子句可以加WHERE子句
3、在ON条件中使用常量过滤谓词来insert所有的行到目标表中,不需要连接源表和目标表
4、UPDATE子句后面可以跟DELETE子句来去除一些不需要的行
语法:
MERGEHINTINTO SCHEMA .TABLE T_ALIAS
USINGSCHEMA . {TABLE | VIEW |SUBQUERY } T_ALIAS
ON (CONDITION)
WHENMATCHEDTHEN MERGE_UPDATE_CLAUSE
WHENNOTMATCHED THEN MERGE_INSERT_CLAUSE / MERGE_DELETE_CLAUSE;
联想:
merge into是特有的功能，相当于在 MSSQL中的
ifexists(...)
updatetable
else
Insertinto table.
mergeinto语法不仅没有if exists语法啰嗦，而且比if exists还要高效很多，常用来在oracle之间同步数据库表。
例子：

1、创建测试表及数据

DROP TABLE PRODUCTS;  
DROP TABLE NEWPRODUCTS;  
create table PRODUCTS  
(  
PRODUCT_ID INTEGER,  
PRODUCT_NAME VARCHAR2(60),  
CATEGORY VARCHAR2(60)  
);  
insert into PRODUCTS values (1501, 'VIVITAR 35MM', 'ELECTRNCS');  
insert into PRODUCTS values (1502, 'OLYMPUS IS50', 'ELECTRNCS');  
insert into PRODUCTS values (1600, 'PLAY GYM', 'TOYS');  
insert into PRODUCTS values (1601, 'LAMAZE', 'TOYS');  
insert into PRODUCTS values (1666, 'HARRY POTTER', 'DVD');  
commit;  
create table NEWPRODUCTS  
(  
PRODUCT_ID INTEGER,  
PRODUCT_NAME VARCHAR2(60),  
CATEGORY VARCHAR2(60)  
);  
insert into NEWPRODUCTS values (1502, 'OLYMPUS CAMERA', 'ELECTRNCS');  
insert into NEWPRODUCTS values (1601, 'LAMAZE', 'TOYS');  
insert into NEWPRODUCTS values (1666, 'HARRY POTTER', 'TOYS');  
insert into NEWPRODUCTS values (1700, 'WAIT INTERFACE', 'BOOKS');  
commit; 

2、匹配更新

MERGE INTO PRODUCTS P  
USING NEWPRODUCTS NP  
ON (P.PRODUCT_ID = NP.PRODUCT_ID)  
WHEN MATCHED THEN  
  UPDATE  
     SET P.PRODUCT_NAME = NP.PRODUCT_NAME,  
         P.CATEGORY     = NP.CATEGORY;  
  
SELECT * FROM PRODUCTS;  
SELECT * FROM NEWPRODUCTS;  

3、不匹配插入

MERGE INTO PRODUCTS P  
USING NEWPRODUCTS NP  
ON (P.PRODUCT_ID = NP.PRODUCT_ID)  
WHEN NOT MATCHED THEN  
  INSERT  
    (PRODUCT_ID  
    ,PRODUCT_NAME  
    ,CATEGORY)  
  VALUES  
    (NP.PRODUCT_ID  
    ,NP.PRODUCT_NAME  
    ,NP.CATEGORY);  
  
SELECT * FROM PRODUCTS;  
SELECT * FROM NEWPRODUCTS; 

4、匹配带where/on更新

MERGE INTO PRODUCTS P  
USING NEWPRODUCTS NP  
ON (P.PRODUCT_ID = NP.PRODUCT_ID)  
WHEN MATCHED THEN  
  UPDATE SET P.PRODUCT_NAME = NP.PRODUCT_NAME WHERE P.CATEGORY = NP.CATEGORY;  
  
MERGE INTO PRODUCTS P  
USING NEWPRODUCTS NP  
ON (P.PRODUCT_ID = NP.PRODUCT_ID AND P.CATEGORY = NP.CATEGORY)  
WHEN MATCHED THEN  
  UPDATE SET P.PRODUCT_NAME = NP.PRODUCT_NAME;  
  
SELECT * FROM PRODUCTS;  
SELECT * FROM NEWPRODUCTS;  
  
SELECT *  
  FROM PRODUCTS A  
 INNER JOIN NEWPRODUCTS B  
    ON A.PRODUCT_ID = B.PRODUCT_ID  
   AND A.CATEGORY = B.CATEGORY;   

5、匹配带where更新、插入

MERGE INTO PRODUCTS P  
USING NEWPRODUCTS NP  
ON (P.PRODUCT_ID = NP.PRODUCT_ID)  
WHEN MATCHED THEN  
  UPDATE  
     SET P.PRODUCT_NAME = NP.PRODUCT_NAME,  
         P.CATEGORY     = NP.CATEGORY  
   WHERE P.CATEGORY = 'DVD'  
WHEN NOT MATCHED THEN  
  INSERT  
    (PRODUCT_ID  
    ,PRODUCT_NAME  
    ,CATEGORY)  
  VALUES  
    (NP.PRODUCT_ID  
    ,NP.PRODUCT_NAME  
    ,NP.CATEGORY) WHERE NP.CATEGORY != 'BOOKS';  
  
SELECT * FROM PRODUCTS;  
SELECT * FROM NEWPRODUCTS;  

6、ON常量表达式

MERGE INTO PRODUCTS P  
USING NEWPRODUCTS NP  
ON (1 = 0)  
WHEN NOT MATCHED THEN  
  INSERT  
    (PRODUCT_ID  
    ,PRODUCT_NAME  
    ,CATEGORY)  
  VALUES  
    (NP.PRODUCT_ID  
    ,NP.PRODUCT_NAME  
    ,NP.CATEGORY) WHERE NP.CATEGORY = 'BOOKS';  
  
SELECT * FROM PRODUCTS;  
SELECT * FROM NEWPRODUCTS;  

7、匹配删除、不匹配插入

MERGE INTO PRODUCTS P  
USING NEWPRODUCTS NP  
ON (P.PRODUCT_ID = NP.PRODUCT_ID)  
WHEN MATCHED THEN  
  UPDATE  
     SET P.PRODUCT_NAME = NP.PRODUCT_NAME,  
         P.CATEGORY     = NP.CATEGORY DELETE  
   WHERE (P.CATEGORY = 'ELECTRNCS')  
WHEN NOT MATCHED THEN  
  INSERT  
    (PRODUCT_ID  
    ,PRODUCT_NAME  
    ,CATEGORY)  
  VALUES  
    (NP.PRODUCT_ID  
    ,NP.PRODUCT_NAME  
    ,NP.CATEGORY);  
  
SELECT * FROM PRODUCTS;  
SELECT * FROM NEWPRODUCTS; 

8、源表为子查询(自联接)

MERGE INTO PRODUCTS P  
USING (SELECT COUNT(*) CO FROM PRODUCTS WHERE PRODUCTS.PRODUCT_ID = 1501) B  
ON (B.CO <> 0)  
WHEN MATCHED THEN  
  UPDATE SET P.PRODUCT_NAME = 'KEBO' WHERE P.PRODUCT_ID = 1501  
WHEN NOT MATCHED THEN  
  INSERT  
    (PRODUCT_ID  
    ,PRODUCT_NAME  
    ,CATEGORY)  
  VALUES  
    (1501  
    ,'KEBO'  
    ,'NBA');  
  
MERGE INTO PRODUCTS P  
USING (SELECT COUNT(*) CO FROM PRODUCTS WHERE PRODUCTS.PRODUCT_ID = 1508) B  
ON (B.CO <> 0)  
WHEN MATCHED THEN  
  UPDATE SET P.PRODUCT_NAME = 'KEBO' WHERE P.PRODUCT_ID = 1508  
WHEN NOT MATCHED THEN  
  INSERT  
    (PRODUCT_ID  
    ,PRODUCT_NAME  
    ,CATEGORY)  
  VALUES  
    (1508  
    ,'KEBO'  
    ,'NBA');  
  
SELECT * FROM PRODUCTS;  

优点：
 —避免了分开更新
 —提高性能并易于使用
 —在数据仓库应用中十分有用
 —使用merge比传统的先判断再选择插入或更新快很多
需要注意的地方：
1、从语法条件上看（ON (join condition)），merge into也是一个危险的语法。如果不慎重使用，会将源表全部覆盖到目标表，既危险又浪费效率，违背了增量同步的原则。所以在设计表结构时，一般每条记录都有“更新时间”的字段，用目标表“最大更新时间”判断源表数据是否有更新和新增的信息。
2、更新的字段，不允许有关联条件的字段（join condition）。比如条件是 A.ID=B.ID，那么使用“SET A.ID=B.ID”将报出一个莫名其妙的提示错误。
 
缺少一个带delete语句选项的示例，暂为补上：

MERGE INTO PRODUCTS P  
USING NEWPRODUCTS NP  
ON (P.PRODUCT_ID = NP.PRODUCT_ID)  
WHEN MATCHED THEN  
  UPDATE  
     SET P.PRODUCT_NAME = NP.PRODUCT_NAME, P.CATEGORY = NP.CATEGORY  
   --WHERE (P.CATEGORY = 'ELECTRNCS')   
  DELETE   
   WHERE (P.CATEGORY = 'ELECTRNCS')  
WHEN NOT MATCHED THEN  
  INSERT  
    (PRODUCT_ID, PRODUCT_NAME, CATEGORY)  
  VALUES  
    (NP.PRODUCT_ID, NP.PRODUCT_NAME, NP.CATEGORY);  
  
SELECT * FROM PRODUCTS;  
SELECT * FROM NEWPRODUCTS;

说明：DELETE语句删除的是满足matched关联on条件，同时也要是update更新内容的子集，否则不会删除任何内容。 

转至：http://blog.csdn.net/nsj820/article/details/5755685