Mybatis 多对多关联关系查询

以学生，课程关系来说：
对于每一个学生和课程的关系是一对多的查询，即一个学生对应多门课程
对于每一门课程和学生是一对多的关系，即一门课程对应多个学生
所以具体实现时仍然采用一对多的方式进行查询

create table t_student(
	id int(6) PRIMARY key auto_increment,
	name varchar(10)
);
CREATE TABLE t_course(
	id int(6) PRIMARY key auto_increment,
	name varchar(5)
);
#多对多关系映射表
CREATE TABLE t_student_course(
	id int(6) PRIMARY key auto_increment,
	sid int(6) REFERENCES t_student(id),
	cid int(6) REFERENCES t_course(id)
);

学生和课程的实体类

public class Course {
    
    
    private Integer id;
    private String name;
    private List<Student> stu;
	//...set get toString
}
public class Student {
    
    
    private Integer id;
    private String name;

    private List<Course> course;
    //...set get toString
}

StudentDAO .java

public interface StudentDAO {
    
    
    //查询学生信息以及选择了哪些课程
   Student queryByID(Integer id);
}

学生类的配置文件（记得配置主配置文件）

<mapper namespace="com.xxx.dao.StudentDAO">
    <resultMap id="stuMap" type="com.xxx.entity.Student">
        <id property="id" column="id"></id>
        <result column="name" property="name"/>
        <collection property="course" javaType="java.util.List" ofType= "com.xxx.entity.Course">
            <id column="cid" property="id"></id>
            <result column="cname" property="name"></result>
        </collection>
    </resultMap>
    <select id="queryByID" parameterType="Int" resultMap="stuMap">
        SELECT
            s.id,
            s.NAME,
            c.NAME cname,
            c.id cid
        FROM
            t_student s,
            t_student_course sc,
            t_course c
        WHERE
            s.id =#{id}
            AND c.id = sc.cid
            AND sc.sid = s.id
    </select>
</mapper>

测试

public class TestStudent {
    
    
    private InputStream in;
    private SqlSession session;
    private StudentDAO studentDAO;

    @Before
    public void init() throws IOException {
    
    
        //1.读取配置文件
        in = Resources.getResourceAsStream("SqlMapConfig.xml");
        //2创建工厂
        SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
        SqlSessionFactory factory = builder.build(in);
        session = factory.openSession();
        studentDAO = session.getMapper(StudentDAO.class);
    }
    @After
    public void destory() throws IOException {
    
    
        //提交事務
        session.commit();
        session.close();
        in.close();
    }

   @Test
    public void TestQueryByID(){
    
    
       Student stu=studentDAO.queryByID(15);
       System.out.println( stu.toString());
       studentDAO.queryByID(12).getCourse().forEach(course->{
    
    
               System.out.println("課程信息"+course);
       });
    }
}

Mybatis 多对多关联关系查询

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