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HomeDatabaseMysql TutorialOracle 的merge into 用法

Merge是一个非常有用的功能,与DB2中的merge into功能几乎一样,与Mysql里的insert into on duplicate key也很类似。MERGE INTO

1.merge into的用途

Merge是一个非常有用的功能,与DB2中的merge into功能几乎一样,与Mysql里的insert into on duplicate key也很类似。MERGE INTO 是Oracle 9i以后才出现的新的功能。简单来说可以是一个“有则更新,无则插入”的功能。

通过Merge into你能够在一个SQL语句中对一个表同时执行inserts和updates操作. 当然是update还是insert是依据于你的指定的条件判断的,Merge into可以实现用B表来更新A表数据,如果A表中没有,则把B表的数据插入A表. 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 。

2.Merge into的基本语法

语法如下 :

1 MERGE [INTO [schema .] table [t_alias]
2 USING [schema .] { table | view | subquery } [t_alias]
3 ON ( condition )
4 WHEN MATCHED THEN [merge_update_clause ]
5 WHEN NOT MATCHED THEN [merge_insert_clause];

通俗点就是如下的语法

1 MERGE INTO [表名] [表别名]
2 USING ( [查询内容] )[或者像表查询一样的语句]
3 ON ([条件表达式] AND [...]...)
4 WHEN MATHED THEN [匹配的update操作]
5 WHEN NOT MATHED THEN [不匹配的insert操作]

举个例子看看:

1 merge into Students st using newStudents nst on (st.id = nst.id)
2 when matched then
3 update set st.name = nst.name
4 when not matched then
5 insert values(nst.id, nst.name, nst.sex)

前面的merger into Students using newStudents 意思是用newStudents表来merge到Students表,匹配关系就是根据on后面的条件子句的内容,这里根据两个表的id来进行匹配,那么匹配上了我们的操作是就是when matched then的子句里的动作了,这里的动作是update set st.name =nst.name, 就是把newStudents里的name,赋值到Students的name里。如果没有匹配上则insert这样的一条语句进去。通过这个简单的小例子,这个merget inot的用法还是比较容易理解的。这里merger into的功能,,好比比较,然后选择更新或者是插入,像是武术套路的一套组合拳。在做merge into的时候,这样同样的情况下,merge的性能是优于同等功能的update/insert语句的。有分析说merge into是单纯的insert/update效率的好几倍,本人没有考究过,不做评论 。

3.Oracle 10g后的改进

在Oracle 10g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

Oracle merge into用法及例子

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