文章目录
1. 多对一的处理
查询所有学生的信息,以及对应的老师的信息。
SQL:
select * from student s, teacher t where s.tid = t.id;
select s.id, s.name, t.name from student s, teacher t where s.tid = t.id;
package com.zk.pojo;
public class Student {
private int id;
private String name;
private Teacher teacher;
}
package com.zk.pojo;
public class Teacher {
private int id;
private String name;
}
方式一:按照查询嵌套处理(子查询)
StudentMapper
public interface StudentMapper {
List<Student> getStudent();
}
StudentMapper.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.zk.dao.StudentMapper">
<!--
思路:
1. 查询所有的学生信息
2. 根据查询出来的学生的tid,查找对应的老师
-->
<select id="getStudent" resultMap="StudentTeacher">
select s.id, s.name, t.name from student s, teacher t where s.tid = t.id;
</select>
<resultMap id="StudentTeacher" type="com.zk.pojo.Student">
<result property="id" column="id"/>
<result property="name" column="name"/>
<!-- <result property="teacher" column="***"/>-->
<!--复杂的属性需要单独处理:
对象:association
集合:collection
-->
<association property="teacher" column="tid" javaType="com.zk.pojo.Teacher" select="getTeacher"/>
</resultMap>
<select id="getTeacher" resultType="com.zk.pojo.Teacher">
select * from Teacher where id = #{id}
</select>
</mapper>
方式二:按照结果嵌套处理(联表查询)
StudentMapper
// 方式二:按照结果嵌套处理(嵌套查询)
List<Student> getStudent2();
StudentMapper.xml
<!-- 方式二 按照结果嵌套处理-->
<select id="getStudent2" resultType="com.zk.pojo.Student">
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.zk.pojo.Student">
<!-- 没起别名,查出来就是默认列;起了别名,对应的列就要为别名 -->
<result property="id" column="sid"/>
<result property="name" column="sname"/>
<association property="teacher" javaType="com.zk.pojo.Teacher">
<result property="name" column="tname"/>
</association>
</resultMap>
2. 一对多的处理
比如一个老师拥有多个学生。获取指定老师下的所有学生及老师的信息,SQL 实现:
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}
实体类
package com.zk.pojo;
import lombok.Data;
import java.util.List;
@Data
public class Teacher {
private int id;
private String name;
private List<Student> students;
}
package com.zk.pojo;
import lombok.Data;
@Data
public class Student {
private int id;
private String name;
private int tid;
}
TeacherMapper
public interface TeacherMapper {
List<Teacher> getTeacher(@Param("tid") int id);
}
TeacherMapper.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.zk.dao.TeacherMapper">
<!-- 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 = {id}
</select>
<resultMap id="TeacherStudent" type="com.zk.pojo.Teacher">
<result property="id" column="tid"/>
<result property="name" column="tname"/>
<!-- javaType=":指定属性的类型,集合中的泛型类型,使用ofType获取-->
<collection property="students" ofType="com.zk.pojo.Student">
<result property="id" column="sid"/>
<result property="name" column="sname"/>
<result property="tid" column="tid"/>
</collection>
</resultMap>
<!-- 按查询嵌套处理 -->
<select id="getTeacher2" resultMap="TeacherStudent2">
select * from teacher where id = #{tid}
</select>
<resultMap id="TeacherStudent2" type="com.zk.pojo.Teacher">
<result property="id" column="id"/>
<result property="name" column="name"/>
<collection property="students" javaType="ArrayList" ofType="com.zk.pojo.Student" select="getStudentByTeacherId" column="id"/>
</resultMap>
<select id="getStudentByTeacherId" resultType="com.zk.pojo.Student">
select * from student where tid = #{tid}
</select>
</mapper>
3. 小结
- association - 多对一
- collection - 一对多
- 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 的类型,泛型中的约束类型