mybatis 复杂sql 一对多 多对一

主意：每次从官网添加 xml映射文件时要修改namespace，xml配置文件别忘添加啊mapper

一、搭建环境

注意：

1、多个javaBean时，xml映射文件，储存到rescourse 文件下， 与dao文件同名和同级下

2、xml映射文件，别忘记修改 namespace

3、xml配置文件的 mapper resource 貌似不能简写(使用*符号)

复杂查询的关系

assocation 用于对象 javaType 作为返回值的关键字

collection 用于集合 ofType 对于数据类型

二、多对一

1、sql语句

    SELECT
        student.id  as sid,
        student.name as sname,
        teacher.name as tname
        FROM student
        left JOIN teacher on student.tid = teacher.id
        

2、接口

package com.wt.dao;

import com.wt.pojo.Student;

import java.util.List;

public interface StudentMapper {
    List<Student> getStudentList();
}

3、xml 映射文件

    <select id="getStudentList" resultMap="isStudentTeacher">
        SELECT
        student.id  as sid,
        student.name as sname,
        teacher.name as tname
        FROM student
        left JOIN teacher on student.tid = teacher.id
    </select>
    <resultMap id="isStudentTeacher" type="Student">
        <result column="sid" property="id"/>
        <result column="sname" property="name"/>
        <!--association 用于对象 javaType 为返回值类型 -->
        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
        </association>
    </resultMap>

4、pojo

package com.wt.pojo;

import lombok.Data;

@Data
public class Student {
    private int id;
    private String name;
    private Teacher teacher;
}

package com.wt.pojo;

import lombok.Data;

@Data
public class Teacher {
    private int id;
    private String name;
}

三、一对多

1、sql语句

SELECT 
teacher.id as tid,
teacher.name as tname,
student.id as sid,
student.name as sname
FROM student 
RIGHT JOIN teacher on student.tid = teacher.id where tid = 1

2、接口类

List<Teacher> getTeacherList(@Param("tid") int id);

3、xml 配置文件

    <select id="getTeacherList" resultMap="teacherStudent">
        SELECT
        teacher.id as tid,
        teacher.name as tname,
        student.id as sid,
        student.name as sname
        FROM student
        RIGHT JOIN teacher on student.tid = teacher.id where tid = #{tid}
    </select>
<!--    collection 集合 类型 ofType-->
    <resultMap id="teacherStudent" type="teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <collection property="students"  ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
        </collection>
    </resultMap>

4、pojo

package com.wt.pojo;

import lombok.Data;

@Data
public class Student {
    private int id;
    private String name;
}

package com.wt.pojo;

import lombok.Data;

import java.util.List;

@Data
public class Teacher {
    private int id;
    private String name;
    private List<Student> students;
}