Mybatis嵌套查询讲解内容的基础

来源：互联网发布：java实训学校编辑：程序博客网时间：2024/05/21 05:38

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

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