(6)MyBatis多对一关联查询

最新推荐文章于 2023-05-15 14:44:01 发布

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最新推荐文章于 2023-05-15 14:44:01 发布

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分类专栏： MyBatis 文章标签： mybatis mysql java

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MyBatis多对一关联查询

类似于一对一查询

修改Student实体类

添加一个MyClass类型的属性

package com.h3c.bean;

public class Student {

    private int id;
    private String name;
    private int age;
    private double score;

	//所在班级
	private MyClass myclass;

    //省略constructor、getter、setter、toString方法
    //注意constructor中的无参构造方法不能少
}

dao层接口

package com.h3c.dao;

public interface studentDao {

	//多对一关联查询
	Student selectStudentById(int id);
	//一对多
	List<Student> selectStudents();
}

StudentMapper.xml

方式一：

<?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.h3c.dao.MyClassDao">

    <resultMap id="StudentMap" type="student">
        <!--student中的基本属性-->
         <id column="id" property="id"/>
        <result column="name" property="name"/>
        <result column="age" property="age"/>
        <result column="score" property="score"/>
        <!--关联属性的映射关系-->
        <association property="myClass" javaType="myClass">
            <id column="id" property="id"/>
            <result column="name" property="name"/>
        </association>
    </resultMap>

    <select id="selectStudentById" resultMap="StudentMap">
        SELECT s.id, s.name, s.age, s.score, c.id, c.name
        FROM t_class c, t_student s
        WHERE c.id = s.cid and c.id=#{id}
    </select>

    <select id="selectStudents" resultMap="StudentMap">
        SELECT s.id, s.name, s.age, s.score, c.id, c.name
        FROM t_class c, t_student s
        WHERE c.id = s.cid
    </select>
</mapper>

方式二：

<?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.h3c.dao.MyClassDao">

    <!-- 多对一查询  -->
    <resultMap id="StudentMap" type="student">
    	<id column="id" property="id"/>
        <result column="name" property="name"/>
        <result column="age" property="age"/>
        <result column="score" property="score"/>
        <!-- 跟一对一一样用association标签,实体类定义的成员,要跟数据库字段名对应上 -->
        <association property="myClass" 
        			 column="cid" 
        			 <!-- 用接口里定义的方法，根据student表中的major字段查出对应数据 -->
        			 select="com.h3c.dao.StudentDao.selectClassById"/> 
    </resultMap>

    <resultMap id="ClassMap" type="myClass">
    	<id column="id" property="id"/>
        <result column="name" property="name"/>
    </resultMap>
    
    <!-- 查全部 -->
    <select id="selectStudentById" resultMap="StudentMap" >
        select id, name, age, score, cid from t_student where id=#{id}
    </select>
    <!-- 根据id查班级  -->
    <select id="selectClassById" parameterType="int" resultMap="ClassMap">
        select id, name from t_class where id=#{id}
    </select>
</mapper>