学步园

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

2、匹配更新

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

3、不匹配插入

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

4、匹配带where/on更新

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

5、匹配带where更新、插入

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

6、ON常量表达式

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

7、匹配删除、不匹配插入

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

8、源表为子查询(自联接)

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