mybatis-一对多查询

准备阶段(一个老师对应多个学生)

1.建表

2.编写实体类

package com.lv.pojo;

import lombok.Data;

@Data
public class Student {
    private int id;
    private String name;
    private int tid;
}

package com.lv.pojo;

import lombok.Data;

import java.util.List;

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

    //一个老师拥有多个学生
    private List<Student> students;
}

3.编写Mapper接口

package com.lv.dao;

public interface StudentMapper {

}

package com.lv.dao;

import com.lv.pojo.Teacher;

import java.util.List;

public interface TeacherMapper {

    //获取老师
    List<Teacher> getTeacher();

}

4.编写Mapper.xml配置文件

<?xml version="1.0" encoding="UTF8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.lv.dao.StudentMapper">

</mapper>

<?xml version="1.0" encoding="UTF8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.lv.dao.TeacherMapper">
    <select id="getTeacher" resultType="Teacher">
        select * from teacher
    </select>

</mapper>

5.在mybatis-config.xml中添加映射

<mappers>
    <mapper class="com.lv.dao.TeacherMapper"/>
    <mapper class="com.lv.dao.StudentMapper"/>
</mappers>

6.编写测试文件进行测试

import com.lv.dao.TeacherMapper;
import com.lv.pojo.Teacher;
import com.lv.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import java.util.List;

public class MyTest {
    @Test
    public void getTeacher(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        List<Teacher> teacherList = mapper.getTeacher();
        for (Teacher teacher : teacherList) {
            System.out.println(teacher);
        }
        sqlSession.close();
    }

}

7.测试结果

方式一:按查询嵌套处理(相当于SQL中的子查询)

1.在TeacherMapper接口中编写方法

Teacher getTeacher3(@Param("tid") int id);

2.编写对应的Mapper.xml文件

<resultMap id="TeacherStudent2" type="Teacher">
    <collection property="students" javaType="ArrayList" ofType="Student" select="getStudentByTeacherId" column="id"/>
</resultMap>
<select id="getTeacher3" resultMap="TeacherStudent2">
    select * from teacher where id = #{tid}
</select>
<select id="getStudentByTeacherId" resultType="Student">
    select * from student where tid = #{tid}
</select>

3.测试

@Test
public void getTeacher3(){
    SqlSession sqlSession = MybatisUtils.getSqlSession();
    TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
    Teacher teacher3 = mapper.getTeacher3(1);
    System.out.println(teacher3);
    sqlSession.close();
}

方式二:按结果嵌套处理(相当于SQL中的连表查询)

1.在TeacherMapper接口中编写方法

//获取指定老师下的所有学生及老师的信息
Teacher getTeacher2(@Param("tid") int id);

2.编写对应的Mapper.xml文件

    <resultMap id="TeacherStudent" type="Teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
<!--        复杂属性单独处理 对象:association 集合:collection
            javaType=""指定的属性类型!
            集合中的泛型信息,我们使用ofType获取
-->
        <collection property="students" ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>
<!--按照结果嵌套查询-->
    <select id="getTeacher2" resultMap="TeacherStudent">
        select s.id sid,s.name sname,t.name tname,t.id tid
        from student s,teacher t
        where s.tid = t.id and t.id = #{tid}
    </select>

3.测试

@Test
public void getTeacher2(){
    SqlSession sqlSession = MybatisUtils.getSqlSession();
    TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
    Teacher teacher2 = mapper.getTeacher2(1);
    System.out.println(teacher2);
    sqlSession.close();
}

注:

1. association 关联 用于一对一和多对一
2. collection 集合 用于一对多
3. javaType 和 ofType 都用来指定对象类型,区别是:javaType 指定的是实里类中的属性,ofType 指定的是映射到list集合属性中实体类的类型