MyBatis学习笔记13 —— 一对多处理

MyBatis学习笔记13 —— 一对多处理

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参考教程B站狂神https://www.bilibili.com/video/BV1NE411Q7Nx

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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

学生类

package com.ylw.pojo;

public class Student {
    private int id;
    private String name;
    private int tid;

    public Student() {
    }

    public Student(int id, String name, int tid) {
        this.id = id;
        this.name = name;
        this.tid = tid;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getTid() {
        return tid;
    }

    public void setTid(int tid) {
        this.tid = tid;
    }

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", tid=" + tid +
                '}';
    }
}

老师类

package com.ylw.pojo;

import java.util.List;

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

    //一个老师拥有多个学生，使用集合
    private List<Student> students;

    public Teacher() {
    }

    public Teacher(int id, String name, List<Student> students) {
        this.id = id;
        this.name = name;
        this.students = students;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public List<Student> getStudents() {
        return students;
    }

    public void setStudents(List<Student> students) {
        this.students = students;
    }

    @Override
    public String toString() {
        return "Teacher{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", students=" + students +
                '}';
    }
}

按照查询嵌套处理（子查询）

接口

Teacher getTeacher2(@Param("tid") int id);

StudentMapper.xml配置文件

    <!--按照查询嵌套处理-->
    <select id="getTeacher2" resultMap="TeacherStudent2">
        select * from mybatis.teacher where id = #{tid}
    </select>

    <resultMap id="TeacherStudent2" type="com.ylw.pojo.Teacher">
        <collection property="students" javaType="ArrayList" ofType="com.ylw.pojo.Student" select="getStudentByTeacherId" column="id"/>
    </resultMap>

    <select id="getStudentByTeacherId" resultType="com.ylw.pojo.Student">
        select * from mybatis.student where tid = #{tid}
    </select>

测试

    @Test
    public void test(){
        SqlSession sqlSession = MyBatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacher2(1);

        System.out.println(teacher);

        sqlSession.close();
    }

按照结果嵌套处理（联表查询）

接口

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

StudentMapper.xml配置文件

    <!--按结果嵌套查询-->
    <select id="getTeacher" 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>

    <resultMap id="TeacherStudent" type="com.ylw.pojo.Teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <!--复杂的属性，我们需要单独处理 对象： association 集合： collection
        javaType="" 指定属性的类型！
        集合中的泛型信息，我们使用ofType获取
        -->
        <collection property="students" ofType="com.ylw.pojo.Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>

测试

    @Test
    public void test(){
        SqlSession sqlSession = MyBatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacher(1);

        System.out.println(teacher);

        sqlSession.close();
    }

小结

1. 关联 - association 【多对一】
2. 集合 - collection 【一对多】
3. javaType & ofType
   1. JavaType 用来指定实体类中属性的类型
   2. ofType 用来指定映射到List或者集合中的 pojo类型，泛型中的约束类型！

注意点：

• 保证SQL的可读性，尽量保证通俗易懂
• 注意一对多和多对一中，属性名和字段的问题！
• 如果问题不好排查错误，可以使用日志 ， 建议使用 Log4j