mybatis-复杂查询（一对多，多对一）

一、多对一（例如多个学生对应一个老师）

1、学生实体

package com.me.domain;

import lombok.Data;

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

2、老师实体

package com.me.domain;

import lombok.Data;

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

方法一：按查询嵌套处理

1、 StudentMapper-getStudentList：

    <select id="getStudentList" resultMap="studentTea">
        select * from student
    </select>
    <resultMap id="studentTea" type="com.me.domain.Student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        
        <association property="teacher" column="tid" javaType="com.me.domain.Teacher" select="getTeacher"/>

    </resultMap>
    <select id="getTeacher" resultType="com.me.domain.Teacher">
        select * from teacher where id = #{tid}
    </select>

2、测试

 @org.junit.Test
    public void getStudentList(){
        SqlSession sqlSession = MyBatisUtils.getSqlSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        List<Student> studentList = mapper.getStudentList();
        for (Student student : studentList) {
            System.out.println(student.toString());
        }
        sqlSession.close();
    }

3、结果

 方法二、案结果嵌套处理（联表查询）

1、 StudentMapper-getStudentList2：

   <select id="getStudentList2" resultMap="studentTea2">
        select s.id sid ,s.name sname ,t.id tid ,t.name tname
        from student s,teacher t
        where s.tid = t.id
    </select>
    <resultMap id="studentTea2" type="com.me.domain.Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="com.me.domain.Teacher">
            <result property="id" column="tid"/>
            <result property="name" column="tname"/>
        </association>
    </resultMap>

2、测试

@org.junit.Test
    public void getStudentList2(){
        SqlSession sqlSession = MyBatisUtils.getSqlSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        List<Student> studentList = mapper.getStudentList2();
        for (Student student : studentList) {
            System.out.println(student.toString());
        }
        sqlSession.close();
    }

3、结果

 二、一对多（一个老师有多个学生）

1、学生实体

package com.me.domain;

import lombok.Data;

@Data
public class Student2 {
    private int id;
    private String name;
    private int tid;
}

2、老师实体

package com.me.domain;

import lombok.Data;

import java.util.List;

@Data
public class Teacher2 {
    private int id;
    private String name;
    private List<Student2> students;
}

方法一：按查询嵌套处理

1、 TeacherMapper-getTeacher2：

    <select id="getTeacher2" resultMap="teacherStu2">
        select * from teacher where id = #{id}
    </select>
    <resultMap id="teacherStu2" type="com.me.domain.Teacher2" >
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <collection property="students" javaType="ArrayList" ofType="com.me.domain.Student2" select="getStudentByTid" column="id"/>
    </resultMap>


    <select id="getStudentByTid" resultType="com.me.domain.Student2">
        select * from student where tid = #{id}
    </select>

2、测试

 @org.junit.Test
    public void getTeacher2(){
        SqlSession sqlSession = MyBatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher2 teacher = mapper.getTeacher2(1);
        System.out.println(teacher.toString());
        sqlSession.close();
    }

3、结果

  方法二、案结果嵌套处理（联表查询）

1、 TeacherMapper-getTeacher：

    <select id="getTeacher" resultMap="teacherStu">
        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 =#{id}
    </select>
    <resultMap id="teacherStu" type="com.me.domain.Teacher2">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <collection property="students" ofType="com.me.domain.Student2">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>

2、测试

@org.junit.Test
    public void getTeacher(){
        SqlSession sqlSession =  MyBatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher2 teacher = mapper.getTeacher(1);
        System.out.println(teacher.toString());
        sqlSession.close();
    }

3、结果

 三、小结

1、集合--collection【一对多】

2、关联--association【多对一】

3、javaType：用来指定实体类中属性的类型

4、ofType：用来指定映射到List或集合中的实体类型，泛型中的约束类型。