Oracle中Merge的使用

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。