怎样解决数据仓库建模中数据一对多的问题

目录

• 1. 什么是一对多和多对一
• 2. 一对多的实现

• 创建实体类
• 按照结果嵌套处理
• 按照查询嵌套处理

• 3. 多对一的实现

• 创建实体类
• 按照结果嵌套处理
• 按照查询嵌套处理

• 注意

1. 什么是一对多和多对一

拿老师和学生举例:
一个老师可以有多个学生, 这就是一对多
多个学生只能有一个老师, 这就是多对一
图中的学生表中的tid对应老师表中的id

2. 一对多的实现

创建实体类

@Data
public class Student {
    private int id;
    private String name;
    private int tid;
}  //学生实体类

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

    //一个老师对应多个学生
    private List<Student> students;
}  //老师实体类

按照结果嵌套处理

在TeacherMapper.xml中:

<select id="getTeacher" resultMap="TeacherStudent">
        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="TeacherStudent" type="Teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <!--collection: 一对多-->
        <collection property="students" ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>

创建Test进行测试

@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();
 }

运行结果

Opening JDBC Connection
Created connection 832279283.
==>  Preparing: 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=? 
==> Parameters: 1(Integer)
<==    Columns: sid, sname, tid, tname
<==        Row: 1, 小明, 1, 秦老师
<==        Row: 2, 小红, 1, 秦老师
<==        Row: 3, 小张, 1, 秦老师
<==        Row: 4, 小李, 1, 秦老师
<==        Row: 5, 小王, 1, 秦老师
<==      Total: 5
Teacher(id=1, name=秦老师, students=[Student(id=1, name=小明, tid=1), Student(id=2, name=小红, tid=1), Student(id=3, name=小张, tid=1), Student(id=4, name=小李, tid=1), Student(id=5, name=小王, tid=1)])
Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@319b92f3]
Returned connection 832279283 to pool.

按照查询嵌套处理

<select id="getTeacher2" resultMap="TeacherStudent2">
        select * from teacher where id=#{tid}
    </select>

    <resultMap id="TeacherStudent2" type="Teacher">
    	<!--如果没有下面这行语句,由于在进行student表查询时使用Teacher类中的id已经被collection占用了,
    		查询出来的结果老师的id会显示为0,所以要提前做一个映射-->
        <result property="id" column="id"/>
        <!--collection: 一对多-->
        <collection property="students" javaType="ArrayList" ofType="Student" select="getStudentByTid" column="id"/>
    </resultMap>

    <select id="getStudentByTid" resultType="Student">
        select * from student where tid=#{tid}
    </select>

创建Test进行测试

@Test
public void test2(){
     SqlSession sqlSession = MybatisUtils.getSqlSession();
     TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);

     Teacher teacher = mapper.getTeacher2(1);
     System.out.println(teacher);

     sqlSession.close();
 }

运行结果

Opening JDBC Connection
Created connection 832279283.
==>  Preparing: select * from teacher where id=? 
==> Parameters: 1(Integer)
<==    Columns: id, name
<==        Row: 1, 秦老师
====>  Preparing: select * from student where tid=? 
====> Parameters: 1(Integer)
<====    Columns: id, name, tid
<====        Row: 1, 小明, 1
<====        Row: 2, 小红, 1
<====        Row: 3, 小张, 1
<====        Row: 4, 小李, 1
<====        Row: 5, 小王, 1
<====      Total: 5
<==      Total: 1
Teacher(id=1, name=秦老师, students=[Student(id=1, name=小明, tid=1), Student(id=2, name=小红, tid=1), Student(id=3, name=小张, tid=1), Student(id=4, name=小李, tid=1), Student(id=5, name=小王, tid=1)])
Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@319b92f3]
Returned connection 832279283 to pool.

3. 多对一的实现

创建实体类

@Data
public class Student {
    private int id;
    private String name;
    //学生关联老师
    private Teacher teacher;
}  //学生实体类
/*==================================================================*/
@Data
public class Teacher {
    private int id;
    private String name;
}  //老师实体类

按照结果嵌套处理

在StudentMapper.xml中

<!--按照结果查询嵌套处理-->
    <select id="getStudent2" resultMap="StudentTeacher2">
        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="StudentTeacher2" type="Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <!--association: 多对一-->
        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
            <result property="id" column="tid"/>
        </association>
    </resultMap>

创建Test进行测试

@Test
public void testStudent2(){
    SqlSession sqlSession = MybatisUtils.getSqlSession();
    StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);

    List<Student> studentList = mapper.getStudent2();
    for (Student student : studentList) {
        System.out.println(student);
    }
    sqlSession.close();
}

运行结果

Opening JDBC Connection
Created connection 1608230649.
==>  Preparing: select s.id sid,s.name sname,t.id tid,t.name tname from student s,teacher t where s.tid=t.id 
==> Parameters: 
<==    Columns: sid, sname, tid, tname
<==        Row: 1, 小明, 1, 秦老师
<==        Row: 2, 小红, 1, 秦老师
<==        Row: 3, 小张, 1, 秦老师
<==        Row: 4, 小李, 1, 秦老师
<==        Row: 5, 小王, 1, 秦老师
<==      Total: 5
Student(id=1, name=小明, teacher=Teacher(id=1, name=秦老师))
Student(id=2, name=小红, teacher=Teacher(id=1, name=秦老师))
Student(id=3, name=小张, teacher=Teacher(id=1, name=秦老师))
Student(id=4, name=小李, teacher=Teacher(id=1, name=秦老师))
Student(id=5, name=小王, teacher=Teacher(id=1, name=秦老师))
Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@5fdba6f9]
Returned connection 1608230649 to pool.

按照查询嵌套处理

<select id="getStudent" resultMap="StudentTeacher">
       select * from student;
    </select>

    <!--查询所有学生信息,根据查询出来的tid,再去查询对应的老师(子查询)-->
    <resultMap id="StudentTeacher" type="Student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <!--association: 多对一-->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
    </resultMap>
    
    <select id="getTeacher" resultType="Teacher">
        select * from teacher where id=#{tid};
    </select>

创建Test进行测试

@Test
public void testStudent(){
    SqlSession sqlSession = MybatisUtils.getSqlSession();
    StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);

    List<Student> studentList = mapper.getStudent();
    for (Student student : studentList) {
        System.out.println(student);
    }
    sqlSession.close();
}

运行结果

Opening JDBC Connection
Created connection 1608230649.
==>  Preparing: select * from student; 
==> Parameters: 
<==    Columns: id, name, tid
<==        Row: 1, 小明, 1
====>  Preparing: select * from teacher where id=?; 
====> Parameters: 1(Integer)
<====    Columns: id, name
<====        Row: 1, 秦老师
<====      Total: 1
<==        Row: 2, 小红, 1
<==        Row: 3, 小张, 1
<==        Row: 4, 小李, 1
<==        Row: 5, 小王, 1
<==      Total: 5
Student(id=1, name=小明, teacher=Teacher(id=1, name=秦老师))
Student(id=2, name=小红, teacher=Teacher(id=1, name=秦老师))
Student(id=3, name=小张, teacher=Teacher(id=1, name=秦老师))
Student(id=4, name=小李, teacher=Teacher(id=1, name=秦老师))
Student(id=5, name=小王, teacher=Teacher(id=1, name=秦老师))
Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@5fdba6f9]
Returned connection 1608230649 to pool.

注意

• 两种查询实现的方式是不一样的, 从日志中就可以看出

• 结果嵌套是一条sql语句直接查出结果, 嵌套是在sql内部完成的
• 查询嵌套是分别运行两条sql语句, 查询后对各自的结果再次进行嵌套

• 集合 -> collection 【一对多】
• 关联 -> association 【多对一】
• javaType 和 ofType

• JavaType 用来指定实体类中属性的类型
• 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！