Mybatis嵌套查询讲解内容的基础

例子是参考Mybatis文档创建的一个简单例子，一共涉及4个表，每个表都有很少的几个属性。

由于说明该内容需要东西太多，专门分成两篇文章，一篇提供表以及基础代码，一篇专门讲内容。

表如下：

Blog表

Blog_user表

Blog_post表

Blog_comments表

代码（其他对象不在这儿贴出来了，请需要的自己​​下载​​）：

BlogMapper.java

public interface BlogMapper {
    BlogDto selectById(String id);
}

BlogMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.easternie.records.dao.BlogMapper" >
  <resultMap id="userMap" type="com.easternie.records.vo.model.BlogUser">
    <id property="userid" column="userid"/>
  <result property="username" column="username" />
  <result property="password" column="password" />
  <result property="userinfo" column="userinfo" />
  </resultMap>
  
  <resultMap id="BlogDtoMap" type="com.easternie.records.vo.model.BlogDto">
    <constructor>
    <idArg column="id" javaType="String"/>    
    </constructor>
    <result property="title" column="title" />
    <result property="url" column="url" />
    <result property="userid" column="userid" />
    <association property="user" resultMap="userMap" />
    <collection property="posts" ofType="com.easternie.records.vo.model.BlogPostDto">
      <id property="postid" column="postid" />
      <!-- <result property="userid" column="userid" /> -->
      <result property="postdate" column="postdate" />
      <result property="postinfo" column="postinfo" />
      <association property="user" resultMap="userMap"/>
      <collection property="comments" ofType="com.easternie.records.vo.model.BlogComments">
        <id property="commentid" column="commentid" />
        <result property="username" column="username2"/>
        <result property="msg" column="msg"/>
      </collection>
    </collection>
  </resultMap>
  
  <select id="selectById" resultMap="BlogDtoMap" parameterType="java.lang.String" >
    select a.id,
       a.title,
       a.userid,
       a.url,
       b.username,
       b.password,
       b.userinfo,
       c.postid,
       c.postdate,
       c.postinfo,
       d.commentid,
       d.username username2,
       d.msg
  from blog a

  left join blog_user b
    on a.userid = b.userid

  left join blog_post c
    on a.userid = c.userid

  left join blog_comments d
    on c.postid = d.postid
 where id = #{id}
  </select>
</mapper>

调用代码：

MyBatis myBatis = MyBatis.getInstance();
    try {
      SqlSessionFactory sessionFactory = myBatis.getSqlSessionFactory();
      SqlSession session = sessionFactory.openSession();
      BlogMapper blogMapper = session.getMapper(BlogMapper.class);
      BlogDto blogDto = blogMapper.selectById("1");
      show(blogDto);
    } catch (Exception e) {
      e.printStackTrace();
    }

下载地址：

​​http://pan.baidu.com/s/1kT7HLdl​​