转自：http://xo-tobacoo.iteye.com/blog/182791
总结下。使用merge比传统的先判断再选择插入或更新快很多。
1）主要功能
提供有条件地更新和插入数据到数据库表中
如果该行存在，执行一个UPDATE操作，如果是一个新行，执行INSERT操作
— 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
2）MERGE语句的语法如下：
MERGE [hint] 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;
还是看例子就知道怎么回事：
MERGE INTO copy_emp c
USING employees e
ON (c.employee_id=e.employee_id)
WHEN MATCHED THEN
UPDATE SET
c.first_name=e.first_name,
c.last_name=e.last_name,
c.department_id=e.department_id
WHEN NOT MATCHED THEN
INSERT VALUES(e.employee_id,e.first_name,e.last_name,
e.email,e.phone_number,e.hire_date,e.job_id,
e.salary,e.commission_pct,e.manager_id,
e.departmetn_id);
MERGE INTO copy_emp c
USING employees e
ON (c.employee_id=e.employee_id)
WHEN MATCHED THEN
UPDATE SET
c.first_name=e.first_name,
c.last_name=e.last_name,
c.department_id=e.department_id
WHEN NOT MATCHED THEN
INSERT VALUES(e.employee_id,e.first_name,e.last_name,
e.email,e.phone_number,e.hire_date,e.job_id,
e.salary,e.commission_pct,e.manager_id,
e.departmetn_id);
3）使用merge的注意事项：
创建测试表：
CREATE TABLE MM (ID NUMBER, NAME VARCHAR2(20));
CREATE TABLE MN (ID NUMBER, NAME VARCHAR2(20));
插入数据
INSERT INTO MM VALUES (1, 'A');
INSERT INTO MN VALUES (1, 'B');
执行：
MERGE INTO MN A
USING MM B
ON(A.ID=B.ID)
WHEN MATCHED THEN
UPDATE SET A.ID = B.ID
WHEN NOT MATCHED THEN
INSERT VALUES(B.ID, B.NAME);
ON(A.ID=B.ID)
报错，原因是on子句的使用的字段不能够用于update，即Oracle不允许更新用于连接的列
修改：
MERGE INTO MN A
USING MM B
ON(A.ID=B.ID)
WHEN MATCHED THEN
UPDATE SET A.NAME = B.NAME
WHEN NOT MATCHED THEN
INSERT VALUES(B.ID, B.NAME);
ON(A.ID=B.ID)
再插入：INSERT INTO MM VALUES (1, 'C');
再执行：
MERGE INTO MN A
USING MM B
ON(A.ID=B.ID)
WHEN MATCHED THEN
UPDATE SET A.NAME = B.NAME
WHEN NOT MATCHED THEN
INSERT VALUES(B.ID, B.NAME);
ON(A.ID=B.ID)
报错，原因无法在源表中获得一组稳定的行
4）更新同一张表的数据。需要注意下细节，因为可能涉及到using的数据集为null,所以要使用count()函数。
MERGE INTO mn a
USING (select count(*) co from mn where mn.ID=4) b
ON (b.co<>0)--这里使用了count和<>，注意下，想下为什么！
WHEN MATCHED THEN
UPDATE
SET a.NAME = 'E'
where a.ID=4
WHEN NOT MATCHED THEN
INSERT
VALUES (4, 'E');
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转自：http://chenxy.blog.51cto.com/729966/823342
Merge into 详细介绍
MERGE语句是 oracle 9i 新增的语法，用来合并 UPDATE 和 INSERT 语句。
通过MERGE语句，根据一张表或子查询的连接条件对另外一张表进行查询，
连接条件匹配上的进行UPDATE，无法匹配的执行INSERT。
这个语法仅需要一次全表扫描就完成了全部工作，执行效率要高于INSERT＋UPDATE。
drop table t;
CREATE TABLE T AS SELECT ROWNUM ID, A.* FROM DBA_OBJECTS A;
drop table t1;
CREATE TABLE T1 AS
SELECT ROWNUM ID, OWNER, TABLE_NAME, CAST('TABLE' AS VARCHAR2(100)) OBJECT_TYPE FROM DBA_TABLES;
MERGE INTO T1 USING T
ON (T.OWNER = T1.OWNER AND T.OBJECT_NAME = T1.TABLE_NAME AND T.OBJECT_TYPE = T1.OBJECT_TYPE)
WHEN MATCHED THEN UPDATE SET T1.ID = T.ID
WHEN NOT MATCHED THEN INSERT VALUES (T.ID, T.OWNER, T.OBJECT_NAME, T.OBJECT_TYPE);
--insert后面不写表示插入全部列
MERGE INTO T1 USING T
ON (T.OWNER = T1.OWNER AND T.OBJECT_NAME = T1.TABLE_NAME)
WHEN MATCHED THEN UPDATE SET T1.ID = T.ID
WHEN NOT MATCHED THEN INSERT VALUES (T.ID, T.OWNER, T.OBJECT_NAME, T.OBJECT_TYPE);
--常见错误，连接条件不能获得稳定的行，可以使用下面的用子查询
MERGE INTO T1
USING (SELECT OWNER, OBJECT_NAME, MAX(ID) ID FROM T GROUP BY OWNER, OBJECT_NAME) T
ON (T.OWNER = T1.OWNER AND T.OBJECT_NAME = T1.TABLE_NAME)
WHEN MATCHED THEN UPDATE SET T1.ID = T.ID
WHEN NOT MATCHED THEN INSERT VALUES (T.ID, T.OWNER, T.OBJECT_NAME);
SELECT ID, OWNER, OBJECT_NAME, OBJECT_TYPE FROM T
MINUS
SELECT * FROM T1;
drop table subs;
create table subs(
msid number(9),
ms_type char(1),
areacode number(3)
);
drop table acct;
create table acct(
msid number(9),
bill_month number(6),
areacode number(3),
fee number(8,2) default 0.00
);
insert into subs values(905310001,0,531);
insert into subs values(905320001,1,532);
insert into subs values(905330001,2,533);
commit;
merge into acct a --操作的表
using subs b on (a.msid=b.msid) --使用原始数据来源的表，并且制定条件，条件必须有括号
when matched then
update set a.areacode=b.areacode
--当匹配的时候，执行update操作，和直接update的语法不一样，不需要制定表名
when not matched then
--当不匹配的时候，执行insert操作，也不需要制定表名，若指定字段插入，
--则在insert后用括号标明，不指定是全部插入
insert(msid,bill_month,areacode) values(b.msid,'200702',b.areacode);
--另外，MERGE语句的UPDATE不能修改用于连接的列，否则会报错
select * from acct;
select * from subs;
--10g新特性，单个操作
merge into acct a
using subs b on(a.msid=b.msid)
when not matched then
--只有单个not matched的时候，只做插入，不做更新，只有单个matched的时候，只做更新操作
insert(a.msid,a.bill_month,a.areacode) values(b.msid,'200702',b.areacode);
update acct set areacode=800 where msid=905320001;
delete from acct where areacode=533 or areacode=531;
insert into acct values(905320001,'200702',800,0.00);
--删除重复行
delete from subs b where b.rowid<(
select max(a.rowid)
from subs a
where a.msid=b.msid and a.ms_type=b.ms_type and a.areacode=b.areacode);
--10g新特性，merge操作之后，只有匹配的update操作才可以，
--用delete where子句删除目标表中满足条件的行。
merge into acct a
using subs b on (a.msid=b.msid)
when MATCHED then
update set a.areacode=b.areacode
delete where (b.ms_type!=0)
when NOT MATCHED then
insert(msid,bill_month,areacode)
values(b.msid,'200702',b.areacode)
where b.ms_type=0;
--10g新特性，满足条件的插入和更新
merge into acct a
using subs b on (a.msid=b.msid)
when MATCHED then
update set a.areacode=b.areacode
where b.ms_type=0
when NOT MATCHED then
insert(msid,bill_month,areacode)
values(b.msid,'200702',b.areacode)
where b.ms_type=0;
select * from subs where ms_type=0;