mybatis映射XML文件详解及实例

mybatis映射XML文件

一个简单的映射文件：

<?xml version="1.0" encoding="UTF-8" ?>

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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