• Mybatis-多对一和一对多

    多对一和一对多

    目录

    • 多个学生,对应一个老师
    • 对于学生而言,关联: 多个学生,关联一个老师 [多对一]
    • 对于老师而言,集合: 一个老师,有很多学生 [一对多]

    1. 复杂表的构建

    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. 测试环境搭建

    1. 导入Lombok

    <dependencies>
    <!-- https://mvnrepository.com/artifact/org.projectlombok/lombok -->
    <dependency>
        <groupId>org.projectlombok</groupId>
        <artifactId>lombok</artifactId>
        <version>1.18.12</version>
    </dependency>
</dependencies>

    2. 新建实体类

    package com.wang.pojo;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

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

    //学生需要关联一个老师(物理外键)
    private Teacher teacher;
}

    package com.wang.pojo;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

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

    3. 建立Mapper接口

    package com.wang.dao;

import com.wang.pojo.Student;

import java.util.List;

public interface StudentMapper {

    //查询所有的学生信息,以及对应的老师的信息
    List<Student> getStudentInfo();
}


    package com.wang.dao;

import com.wang.pojo.Teacher;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;

public interface TeacherMapper {

    @Select("select * from teacher where id = #{tid}")
    Teacher getTeacher(@Param("tid") int id);

}

    4. 建立Mapper.xml文件

    <?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.wang.dao.StudentMapper">



</mapper>

    <?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.wang.dao.TeacherMapper">



</mapper>

    5. 在核心配置文件汇总绑定Mapper接口或者文件

    <?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<!--configuration 核心配置文件-->
<configuration>

    <!--引入外部配置文件,由于此处都在resource目录下,直接写文件名-->
    <properties resource="db.properties"/>

    <!--标准的日志工厂实现-->
    <settings>
        <setting name="logImpl" value="STDOUT_LOGGING"/>
    </settings>


    <!--可以给实体类起别名-->
    <typeAliases>
        <typeAlias type="com.wang.pojo.Teacher" alias="Teacher"/>
    </typeAliases>

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${pwd}"/>
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <mapper resource="com/wang/dao/StudentMapper.xml"/>
        <!--由于使用了注解,Teacher要用class关联接口!-->
        <mapper class="com.wang.dao.TeacherMapper"/>
    </mappers>

</configuration>

    6. 测试查询是否成功

    import com.wang.dao.TeacherMapper;
import com.wang.pojo.Teacher;
import com.wang.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

public class MyTest {

    @Test
    public void TestGetTeacher() {
        SqlSession sqlSession = MybatisUtils.getSqlSession();

        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacher(1);
        System.out.println(teacher);

        sqlSession.close();
    }
}

    3. 多对一处理

    1. 按照查询嵌套处理(子查询)

    <?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.wang.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,寻找对应的老师
    -->
    <select id="getStudentInfo" 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>


</mapper>

    2. 按照结果嵌套处理(连表查询)

    <!--===========================================================-->
<!--按照结果嵌套处理-->
<select id="getStudentInfo2" resultMap="StudentTeacher2">
    select s.id as sid, s.name as sname, t.name as tname
    from student as s, teacher as 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>

    4. 一对多处理

    1. 环境搭建,修改实体类

    package com.wang.pojo;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

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

    package com.wang.pojo;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.List;

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

    //一个老师拥有多个学生
    private List<Student> students;
}

    2. 按照结果嵌套处理

    <!--按结果嵌套查询-->
<select id="getTeacherById" resultMap="TeacherStudent">
    select s.id as sid, s.name as sname, t.name as tname, t.id as tid
    from student as s
    inner join teacher as t
    on s.tid = t.id and tid = #{tid};
</select>
<!--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!
    对于集合中的泛型信息,我们使用ofType获取!
-->
<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"/>
        <result property="tid" column="tid"/>
    </collection>
</resultMap>

    3. 按照查询嵌套处理

    <select id="getTeacherById2" resultMap="TeacherStudent2">
    select *
    from teacher
    where id = #{tid};
</select>
<resultMap id="TeacherStudent2" type="Teacher">
    <collection property="students" javaType="ArrayLi st" ofType="Student" select="getStudentByTeacherId" column="id"/>
</resultMap>

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

    此种方式,用于查询的字段必须显示指定才能正确显示!

    两种显式指定的方式:

    1. 在SQL语句中指定
    2. 在结果集映射中使用result标签通过property和column指定

    5. 小结

    1. association 关联 多对一
    2. collection 集合 一对多
    3. javaType & ofType
      1. javaType 用来指定实体类中属性的类型
      2. 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

    6. 注意点

    • 保证SQL的可读性,尽量保证通俗易懂
    • 注意一对多和多对一中,属性名和字段的问题!
    • 如果问题不好排查错误,可以使用日志,建议使用Log4j
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  • 原文地址：https://www.cnblogs.com/wang-sky/p/13595407.html
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