myBatis学习笔记（4）——一对一关联表查询

需求

现有两个实体，student和teacher，假设一个学生对应一个老师，他们是一对一的关系，我们想要在查询学生的时候，也能得到该学生的老师的信息

准备

数据库两张表：student和teacher

实体类

package com.bank.entities;public class Teacher {    private int teacherId;    private String teacherName;    public int getTeacherId() {        return teacherId;    }    public void setTeacherId(int teacherId) {        this.teacherId = teacherId;    }    public String getTeacherName() {        return teacherName;    }    public void setTeacherName(String teacherName) {        this.teacherName = teacherName;    }    @Override    public String toString() {        return "Teacher [teacherId=" + teacherId + ", teacherName="                + teacherName + "]";    }}

package com.bank.entities;public class Student {    private int stuId;    private String stuName;    private Teacher teacher;    public int getStuId() {        return stuId;    }    public void setStuId(int stuId) {        this.stuId = stuId;    }    public String getStuName() {        return stuName;    }    public void setStuName(String stuName) {        this.stuName = stuName;    }    public Teacher getTeacher() {        return teacher;    }    public void setTeacher(Teacher teacher) {        this.teacher = teacher;    }    @Override    public String toString() {        return "Student [stuId=" + stuId + ", stuName=" + stuName                + ", teacher=" + teacher + "]";    }}

注意：在Student类中，有一个属性是Teacher，而不是teacherId

问题

很显然，我们需要关联student和teacher两张表，那么，多表查询的结果集，如何填充到student实体类中的同时，也把teacher属性中的属性对应填充好呢？

方案一

    <!-- 方法一  -->    <!-- 配置一个映射关系 -->    <resultMap type="Student" id="studentResultMap1">        <id property="stuId" column="stu_id"/>        <result property="stuName" column="stu_name"/>        <!-- association是用来处理一对一关系的结果映射 -->        <association property="teacher" javaType="Teacher">            <id property="teacherId" column="teacher_id"/>            <result property="teacherName" column="teacher_name"/>        </association>    </resultMap>    <!-- 查询结果集引用配好的映射关系 -->    <select id="getStudent1" parameterType="int"        resultMap="studentResultMap1">        select S.stu_id,S.stu_name,T.teacher_id,T.teacher_name            from student S,teacher T            where stu_id=#{id}            and   S.stu_id=T.teacher_id    </select>

这种方法是直接关联两张表，执行一次查询

方案二

<!--         方法二  :执行两次查询    -->    <select id="getStudent2" parameterType="int" resultMap="studentResultMap2">        select * from student where stu_id=#{id}    </select>    <select id="getTeacher" parameterType="int" resultType="Teacher">        select teacher_id teacherId,teacher_name teacherName from teacher where teacher_id=#{id}    </select>    <resultMap type="Student" id="studentResultMap2">        <id property="stuId" column="stu_id"/>        <result property="stuName" column="stu_name"/>        <!-- 使用查询出来的teacher_id字段的值，作为参数调用 getTeacher这个查询-->        <association property="teacher" column="teacher_id" select="getTeacher">        </association>    </resultMap>

这种方法执行了两次查询：先查询学生信息，然后再用查询出来的teacherId去做第二次查询

两种方案得到的结果是一致的