Oracle MERGE INTO語法探究

1、MERGE INTO 的用途：
   MERGE INTO 是Oracle 9i新增的語法，在10g時得到補充，用來合并UPDATE和INSERT語句，根據一張表或子查詢與另一張表進行連接查詢，連接條件匹配就進行

UPDATE，不匹配就進行INSERT，這個語法僅需要一次全表掃描就可以完成全部工作，執行效率會比單純的UPDATE+INSERT高，具體應用可用于表之間的同步。
2、MERGE INTO 的語法：
語法結構：
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;

語法說明：

MERGE INTO [表名] [別名]        --需要更新的目標表

USING ( 子查詢/表名/視圖)[別名] --源表

ON ([連接條件] AND [...]...)    --連接條件/更新條件

WHEN MATHED THEN UPDATE SET [...]  --如果匹配，更新表記錄，若只作更新出來，下面的INSERT部分可以去掉

WHEN NOT MATHED THEN INSERT VALUES() [...]  --如果不匹配，插入表記錄
3、MERGE INTO 演示：
  1> 創建測試表及數據：
     --以表YAG1作為源表，表YAG2作為更新的目標表
     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中某條記錄的OBJECT_NAME，創造符合UPDATE的條件，
     SQL> UPDATE YAG1 SET OBJECT_NAME='AAAAA' WHERE OBJECT_NAME='T_CAT';
  2>MERGE INTO 更新前兩表的記錄對比：
     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> 執行下面MERGE INTO 語句：
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 更新后兩表的記錄對比：
     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
.