一对多查询

比如：一个老师拥有多个学生！

对于老师而言就是一对多的关系!

实体类

Student

package com.kuang.pojo;

import lombok.Data;

@Data
public class Student {
    private int id;
    private String name;
    //学生需要关联一个老师
    private int tid;

}

Teacher

package com.kuang.pojo;

import lombok.Data;

import java.util.List;

@Data
public class Teacher {
    private int id;
    private String name;
    private List<Student> students;





}

接口

package com.kuang.dao;

import com.kuang.pojo.Teacher;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;

import java.util.List;

public interface TeacherMapper {
   List<Teacher> getTeacher();

   //一对多  一个老师对应多个学生
   //结果嵌套查询
   Teacher getTeacherStudent(@Param("tid") int id);
   //嵌套子查询
   Teacher getTeacherStudent2(@Param("tid") int id);


}

结果集嵌套映射 

<!--结果嵌套映射-->
    <select id="getTeacherStudent" resultMap="STselect">
        SELECT s.id sid,s.`name` sname, t.id tid, t.`name` tname from student s,teacher t WHERE s.tid=t.id AND t.id=#{tid}
    </select>
    <resultMap id="STselect" type="Teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <collection property="students" ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>

        </collection>
    </resultMap>

 子查询嵌套

<!-- 子查询嵌套   -->
    <select id="getTeacherStudent2" resultMap="getTeacherStu">
        select * from mybatis.teacher where id =#{tid}
    </select>
    <resultMap id="getTeacherStu" type="Teacher">
        <result property="id" column="id"/>
        <collection property="students" javaType="ArrayList" ofType="Student" select="stu" column="id"></collection>
    </resultMap>
    <select id="stu" resultType="Student">
        select * from mybatis.Student where tid =#{tid}
    </select>

 测试类

package com.kuang.dao;

import com.kuang.pojo.Teacher;
import com.kuang.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import java.util.List;

public class MyTest {
    @Test
    public void getTest(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        List<Teacher> teacher = mapper.getTeacher();
        for (Teacher teacher1 : teacher) {
            System.out.println(teacher1);
        }

        sqlSession.close();
    }

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

    @Test
    public  void  getST2(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacherStudent = mapper.getTeacherStudent2(1);
        System.out.println(teacherStudent);
        sqlSession.close();

    }

}

 小结

关联 association   多对一

集合 collection   一对多

JavaType  用来指定实体类中属性的类型

OfType  泛型中的约束类型

注意点：保证SQL的可读性！

注意一对多和多对一中属性名和字段的问题！

如果问题不好排查，建议使用日志 ，建议使用Log4j

避免慢SQL