Oracle MERGE INTO语法探究
1、MERGE INTO 的用途：
MERGE INTO 是Oracle 9i新增的语法，在10g时得到补充，用来合并UPDATE和INSERT语句，根据一张表或子查询与另一张表进行连接查询，连接条件匹配就进行
UPDATE，不匹配就进行INSERT，这个语法仅需要一次全表扫描就可以完成全部工作，执行效率会比单纯的UPDATE+INSERT高，具体应用可用于表之间的同步。
2、MERGE INTO 的语法：
语法结构：
MERGE [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;
语法说明：
MERGE INTO [表名] [别名] --需要更新的目标表
USING ( 子查询/表名/视图)[别名] --源表
ON ([连接条件] AND [...]...) --连接条件/更新条件
WHEN MATHED THEN UPDATE SET [...] --如果匹配，更新表记录，若只作更新出来，下面的INSERT部分可以去掉
WHEN NOT MATHED THEN INSERT VALUES() [...] --如果不匹配，插入表记录
3、MERGE INTO 演示：
1> 创建测试表及数据：
--以表YAG1作为源表，表YAG2作为更新的目标表
CREATE TABLE YAG1 AS SELECT OBJECT_NAME,oOBJECT_ID FROM USER_OBJECTS WHERE ROWNUM<=10;
CREATE TABLE YAG2 AS SELECT OBJECT_NAME,oOBJECT_ID FROM USER_OBJECTS WHERE ROWNUM<=5;
--修改表YAG1中某条记录的OBJECT_NAME，创造符合UPDATE的条件，
SQL> UPDATE YAG1 SET OBJECT_NAME='AAAAA' WHERE OBJECT_NAME='T_CAT';
2>MERGE INTO 更新前两表的记录对比：
SQL> SELECT A.OBJECT_ID,A.OBJECT_NAME,B.OBJECT_NAME FROM YAG1 A,YAG2 B WHERE A.OBJECT_ID=B.OBJECT_ID(+) ORDER BY 1;
A.OBJECT_ID A.OBJECT_NAME B.OBJECT_NAME
------------ ---------------- -----------------
46366 AAAAA T_CAT
46367 SUM_STRING SUM_STRING
46368 ARRAYLIST ARRAYLIST
46369 TYSKZ_SJDX TYSKZ_SJDX
46370 TYSKZ_SJXMGX TYSKZ_SJXMGX
46371 PARAOBJECT
46372 T_LINK
46373 STR_SPLIT
46374 SPLIT_TYPE
46375 SYS_PLSQL_95487_9_1
3> 执行下面MERGE INTO 语句：
MERGE INTO YAG2 A
USING YAG1 B
ON (A.OBJECT_ID = B.OBJECT_ID)
WHEN MATCHED THEN
UPDATE SET A.OBJECT_NAME = B.OBJECT_NAME
WHEN NOT MATCHED THEN
INSERT VALUES (B.OBJECT_NAME, B.OBJECT_ID);
COMMIT;
4> MERGE INTO 更新后两表的记录对比：
SQL> SELECT A.OBJECT_ID,A.OBJECT_NAME,B.OBJECT_NAME FROM YAG1 A,YAG2 B WHERE A.OBJECT_ID=B.OBJECT_ID(+) ORDER BY 1;
A.OBJECT_ID A.OBJECT_NAME B.OBJECT_NAME
------------ ---------------- -----------------
46366 AAAAA AAAAA
46367 SUM_STRING SUM_STRING
46368 ARRAYLIST ARRAYLIST
46369 TYSKZ_SJDX TYSKZ_SJDX
46370 TYSKZ_SJXMGX TYSKZ_SJXMGX
46371 PARAOBJECT PARAOBJECT
46372 T_LINK T_LINK
46373 STR_SPLIT STR_SPLIT
46374 SPLIT_TYPE SPLIT_TYPE
46375 SYS_PLSQL_95487_9_1 SYS_PLSQL_95487_9_1