mybatis映射XML文件的示例分析

这篇文章主要介绍mybatis映射XML文件的示例分析，文中介绍的非常详细，具有一定的参考价值，感兴趣的小伙伴们一定要看完！

mybatis映射XML文件

一个简单的映射文件：

当然这个文件中没有任何的元素

The Mapper XML files have only a few first class elements :

1. cache - Configuration of the cache for a given namespace.

2. cache-ref - Reference to a cache configuration from another namespace.

3. resultMap - The most complicated and powerful element that describes how to load your objects from the database result sets.

4. sql - A reusable chunk of SQL that can be referenced by other statements.

5. insert - A mapped INSERT statement.

6. update - A mapped UPDATE statement.

7. delete - A mapped DELETE statement.

8. select - A mapped SELECT statement.
   

select

简单的例子：

 SELECT * FROM PERSON WHERE ID = #{id}

select也有很多属性可以让你配置：

insert, update and delete
语句：
 insert into Author (id,username,password,email,bio) values (#{id},#{username},#{password},#{email},#{bio}) update Author set  username = #{username},  password = #{password},  email = #{email},  bio = #{bio} where id = #{id} delete from Author where id = #{id}
f your database supports auto-generated key fields (e.g. MySQL and SQL Server)，上面的插入语句可以写成：
 insert into Author (username,password,email,bio) values (#{username},#{password},#{email},#{bio})
如果你的数据库还支持多条记录插入，可以使用下面这个语句：
 insert into Author (username, password, email, bio) values   (#{item.username}, #{item.password}, #{item.email}, #{item.bio}) 
sql
这个element可以定义一些sql代码的碎片，然后在多个语句中使用，降低耦合。比如：
 ${alias}.id,${alias}.username,${alias}.password 
然后在下面的语句中使用：
 select  ,   from some_table t1  cross join some_table t2

Result Maps

官网给了个最最复杂的例子

大体意思呢就是一个博客系统有一个作者，很多博文，博文中有一个作者，很多评论，很多标签（包括了一对多，一对一）

 select    B.id as blog_id,    B.title as blog_title,    B.author_id as blog_author_id,    A.id as author_id,    A.username as author_username,    A.password as author_password,    A.email as author_email,    A.bio as author_bio,    A.favourite_section as author_favourite_section,    P.id as post_id,    P.blog_id as post_blog_id,    P.author_id as post_author_id,    P.created_on as post_created_on,    P.section as post_section,    P.subject as post_subject,    P.draft as draft,    P.body as post_body,    C.id as comment_id,    C.post_id as comment_post_id,    C.name as comment_name,    C.comment as comment_text,    T.id as tag_id,    T.name as tag_name from Blog B    left outer join Author A on B.author_id = A.id    left outer join Post P on B.id = P.blog_id    left outer join Comment C on P.id = C.post_id    left outer join Post_Tag PT on PT.post_id = P.id    left outer join Tag T on PT.tag_id = T.id where B.id = #{id}                                                

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