多对一和一对多

10.多对一

准备sql

CREATE TABLE `teacher` (
`id` INT(10) NOT NULL,
`name` VARCHAR(30) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8;
INSERT INTO teacher(`id`, `name`) VALUES (1, '李老师');
CREATE TABLE `student` (
`id` INT(10) NOT NULL,
`name` VARCHAR(30) DEFAULT NULL,
`tid` INT(10) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `fktid` (`tid`),
CONSTRAINT `fktid` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8;
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('1', '小明', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('2', '小红', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('3', '小张', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('4', '小李', '1');
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('5', '小王', '1');

做法： select s.id,s.name,t.name from student s,teacher t where s.tid=t.id;

用mybatis实现，类似子查询 ，法一：

<mapper namespace="com.kuang.dao.StudentMapper">
    <!--思路：1.查询所有学生的信息.2.根据查询出来的学生的tid，寻找对应的老师-->
    <select id="getStudent" resultMap="StudentTeacher">
select * from student
</select>
  《其中id表示他的标识，type表示那种类作为其映射的类》
    <resultMap id="StudentTeacher" type="Student">
        <result property="name" column="name"/>
        <!--复杂的属性，我们需要单独处理对像：association 集合：collection-->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
    </resultMap>
    <select id="getTeacher" resultType="com.kuang.pojo.Teacher">
        select * from teacher where id=#{id}
    </select>
​
</mapper>

法二：

  <select id="getStudent2" resultMap="StudentTeacher2">
        select s.id sid,s.name sname,t.name tname
        from student s,teacher t
         where s.tid=t.id;
    </select>
    <resultMap id="StudentTeacher2" type="com.kuang.pojo.Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="com.kuang.pojo.Teacher">
            <result property="name" column="tname"/>
        </association>
    </resultMap>

注意 ： <!--复杂的属性，我们需要单独处理对像：association 集合：collection-->

一对多

实体类

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

<mapper namespace="com.kuang.dao.TeacherMapper">
    <select id="getTeacher" resultMap="TeacherStudent">
    select s.id sid,s.name sname,t.name tname,t.id tid
    from  teacher t,student s
    where s.id=t.id and t.id=#{tid}
    </select>
    <resultMap id="TeacherStudent" type="com.kuang.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.kuang.pojo.Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>
</mapper>

小结：

1.关联-association【多对一】

2.集合-collection【一对多】

JavaType & 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pojo类型，反型中的约束类型