按照查询嵌套处理(稍微复杂点)

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.mybatisstudy.dao.StudentMapper">

    <!--
     思路:
        1.查询所有的学生信息
        2.根据查询出来的学生的tid 寻找对应的老师
    -->

    <select id="getStudent" resultMap="StudentTeacher">
        select * from student
<!--        select s.id,s.name,t.name from student s,teacher  t where s.tid=t.id-->
    </select>
    <resultMap id="StudentTeacher" type="com.mybatisstudy.pojo.Student">
        <result property="id" column="id"></result>
        <result property="name" column="name"></result>
        <!--复杂的属性 需要单独处理
            对象:association
            集合:collection
        -->
        <association property="teacher" column="tid" javaType="com.mybatisstudy.pojo.Teacher" select="getTeacher"></association>
    </resultMap>

    <select id="getTeacher" resultType="com.mybatisstudy.pojo.Teacher">
        select * from teacher where id = #{id}
    </select>
</mapper>

按照结果嵌套处理(偏简单,推荐使用)

<!--按照结果嵌套处理-->
<select id="getStudentNew" resultMap="StudentNew">
    select s.id sid,s.name sname,t.name tname from student s,teacher  t where s.tid=t.id;
</select>

<resultMap id="StudentNew" type="com.mybatisstudy.pojo.Student">
    <result property="id" column="sid"></result>
    <result property="name" column="sname"></result>
    <association property="teacher" javaType="com.mybatisstudy.pojo.Teacher">
        <result property="name" column="tname"></result>
    </association>
</resultMap>

Mysql多对一查询方式:

• 子查询
• 联表查询