多对一处理

测试环境搭建（参考之前）

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Teacher {
    private int id;
    private String name;
}

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Student {
    private int id;
    private String name;
    private Teacher teacher;
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">


<!-- 核心配置文件 -->
<!-- 思路：1.查询多有学生的信息
          2.根据查询出来的学生的tid，寻找对应的老师！-->

<mapper namespace="com.sun.dao.StudentMapper">
    <select id="getStudentList" resultMap="StudentTeacher">
        select * from student
    </select>
    <resultMap id="StudentTeacher" type="Student">
        <result property="id" column="id"></result>
        <result property="name" column="name"></result>
        <!-- 复杂的属性，我们需要单独处理   对象 ：association  集合：collection -->
        <association property="teacher" column="tid" javaType="Teacher" select="getteacher"></association>
    </resultMap>

    <select id="getteacher" resultType="Teacher">
        select * from teacher
    </select>



    <select id="getStudentList2" resultMap="StudentMap" >
        select a.id sid,a.name sname,a.tid ,b.name teachername from student a, teacher b where a.tid=b.id
    </select>
    <resultMap id="StudentMap" type="Student">
        <result property="id" column="sid"></result>
        <result property="name" column="sname"></result>
        <association property="teacher" javaType="Teacher">
            <result property="id" column="tid"></result>
            <result property="name" column="teachername"></result>
        </association>
    </resultMap>
</mapper>

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">


<!-- 核心配置文件 -->
<mapper namespace="com.sun.dao.TeacherMapper">

</mapper>

package com.sun.dao;

import com.sun.pojo.Student;

import java.util.List;
public interface StudentMapper {
    List<Student> getStudentList();
    List<Student> getStudentList2();
}

package com.sun.dao;

import com.sun.pojo.Teacher;
import org.apache.ibatis.annotations.Select;

import java.util.List;

public interface TeacherMapper {
    @Select("select * from Teacher")
    List<Teacher> getTeacher();
}

mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">


<!-- 核心配置文件 -->
<configuration>
    <properties resource="database.properties"></properties>
    <settings>
        <setting name="logImpl" value="LOG4J"/>
    </settings>
    <!-- 可以给实体类起别名 -->
    <typeAliases>
        <package name="com.sun.pojo"/>
    </typeAliases>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}"/>
                <property name="url" value="${jdbc.url}"/>
                <property name="username" value="${jdbc.username}"/>
                <property name="password" value="${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper class="com.sun.dao.TeacherMapper"></mapper>
        <mapper class="com.sun.dao.StudentMapper"></mapper>
    </mappers>
</configuration>

===============================================================
总结

• 按查询嵌套处理

<select id="getStudentList" resultMap="StudentTeacher">
        select * from student
    </select>
    <resultMap id="StudentTeacher" type="Student">
        <result property="id" column="id"></result>
        <result property="name" column="name"></result>
        <!-- 复杂的属性，我们需要单独处理   对象 ：association  集合：collection -->
        <association property="teacher" column="tid" javaType="Teacher" select="getteacher"></association>
    </resultMap>

    <select id="getteacher" resultType="Teacher">
        select * from teacher
    </select>

• 按结果嵌套处理

<select id="getStudentList2" resultMap="StudentMap" >
        select a.id sid,a.name sname,a.tid ,b.name teachername from student a, teacher b where a.tid=b.id
    </select>
    <resultMap id="StudentMap" type="Student">
        <result property="id" column="sid"></result>
        <result property="name" column="sname"></result>
        <association property="teacher" javaType="Teacher">
            <result property="id" column="tid"></result>
            <result property="name" column="teachername"></result>
        </association>
    </resultMap>

github地址：https://github.com/sunmaio159/mybatis-study