多对一处理
多个学生对应一个老师 【关联 association】
1.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, '秦老师');
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');
2.环境搭建
环境搭建好后,应测试搭建是否成功!!
1.创建一个新的maven子工程、pom.xml中添加Lombok依赖
2.mybatis-config.xml、db.properties 配置文件
3.新建包名 com.utils、com.pojo、com.dao
3.MyBatisUtils.java 工具类、Student.java、Teacher.java实体类
4.dao层接口 StudentMapper.java 、TeacherMapper.java
5.dao层xml实现 StudentMapper.xml、TeacherMapper.xml
5.测试类 MyTest.java
实体类
注意:因为关系的不同,多对一和一对多涉及的实体类的属性也有所不同!!!
Student.java
@Data
public class Student{
private int id;
private String name;
//学生需要关联一个老师
private Teacher teacher;
}
Teacher.java
@Data
public class Teacher{
private int id;
private String name;
}
3.按照查询嵌套处理~子查询
<!--
思路:
1. 查询所有的学生信息
2. 根据查询出来的学生的tid,寻找对应的老师! 子查询
-->
<select id="getStudent" resultMap="StudentTeacher">
select * from student
</select>
<resultMap id="StudentTeacher" type="Student">
<result property="id" column="id"/>
<result property="name" column="name"/>
<!--复杂的属性,我们需要单独处理 对象: association 集合: collection -->
<association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
</resultMap>
<select id="getTeacher" resultType="Teacher">
select * from teacher where id = #{id}
</select>
4.按照结果嵌套查询~联表查询
<!--按照结果嵌套处理-->
<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="Student">
<result property="id" column="sid"/>
<result property="name" column="sname"/>
<association property="teacher" javaType="Teacher">
<result property="name" column="tname"/>
</association>
</resultMap>
回顾Mysql 多对一查询方式:
- 子查询
- 联表查询
一对多处理
一个老师有多个学生【集合 collection】
sql、环境搭建同上。
实体类
Student.java
@Data
public class Student{
private int id;
private String name;
private int tid;
}
Teacher.java
@Data
public class Teacher{
private int id;
private String name;
//一个老师拥有多个学生
private List<Student> students;
}
1.按照结果嵌套处理
<!--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="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="Student">
<result property="id" column="sid"/>
<result property="name" column="sname"/>
<result property="tid" column="tid"/>
</collection>
</resultMap>
2.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="Teacher">
<collection property="students" javaType="ArrayList" ofType="Student" select="getStudentByTeacherId" column="id"/>
</resultMap>
<select id="getStudentByTeacherId" resultType="Student">
select * from mybatis.student where tid = #{tid}
</select>
小结
- 关联 - association 【多对一】
- 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或者集合中的 pojo类型,泛型中的约束类型!
- 两种处理方式,都挺难的,多练就好,但是更推荐的是按照结果查询,因为sql语句可以验证