MyBatis执行多对一语句查询

1、创建maven工程，搭建mybatis框架环境

mybatis-config.xml配置文件以及db。properties配置

<?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>
    <!--    引入外部配置文件-->
    <properties resource="db.properties"/>


<!--    配置日志文件，注意不要写错-->
    <settings>
<!--        标准日志工厂-->
        <setting name="logImpl" value="STDOUT_LOGGING"/>
    </settings>


    <typeAliases>
<!--        1、可以给实体类起别名,实体类少使用-->
<!--        <typeAlias type="pojo.User" alias="User"/>-->
<!--        2、可以给实体类起别名,默认别名为类名首字母小写
            若有注解，则以注解为别名-->
        <package name="pojo"/>
    </typeAliases>


    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
<!--            连接数据库，数据库使用的是mysql8.0-->
            <dataSource type="POOLED">
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>

    </environments>
<!--注册StudentMapper.xml-->
    <mappers>
        <mapper class="dao.StudentMapper"/>
    </mappers>


</configuration>
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数据库使用使用MySQL8.0

driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://localhost:3306/表名?useSSL=false&allowPublicKeyRetrieval=true&serverTimezone=UTC
username=数据库用户名
password=数据库密码
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pom.xml依赖

  <dependencies>
<!--        mysql驱动-->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.11</version>
        </dependency>
<!--mybatis所需jar包-->
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.2</version>
        </dependency>

        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
        </dependency>
    </dependencies>
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2、创建stdent和teacher数据表

CREATE TABLE student (

id int NOT NULL AUTO_INCREMENT,

name varchar(20) ，

tid int NULL DEFAULT NULL,

PRIMARY KEY (id) USING BTREE,

INDEX fktid(tid) USING BTREE,

CONSTRAINT fktid FOREIGN KEY (tid) REFERENCES teacher (id) ON DELETE RESTRICT ON UPDATE RESTRICT

)

CREATE TABLE teacher (

id int NOT NULL AUTO_INCREMENT,

name varchar(20) ,

PRIMARY KEY (id) USING BTREE

)

3、创建对应实体类

package pojo;




public class Student {
    private int id;
    private String name;
    //学生关联一个老师
    private Teacher teacher;

    public Student() {
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Teacher getTeacher() {
        return teacher;
    }

    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }

    public Student(int id, String name, Teacher teacher) {
        this.id = id;
        this.name = name;
        this.teacher = teacher;
    }
}
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package pojo;

import lombok.Data;


public class Teacher {
    private int id;
    private String name;

    public Teacher() {
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Teacher(int id, String name) {
        this.id = id;
        this.name = name;
    }
}
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4、创建StudentMapper接口以

package dao;

import pojo.Student;

import java.util.List;

public interface StudentMapper {

    //多个学生对一个老师
    //查找所有学生信息以及对应教师
    List<Student> getStudents();

}
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5、创建StudentMapper.xml配置文件，写入sql语句以及配置

方式一按结果嵌套查询

    <select id="getStudents" resultMap="Students">
        select s.id sid,s.name sname,t.name tname
        from student s,teacher t
        where s.tid=t.id
    </select>

    <resultMap id="Students" type="Student">
<!--        property对应实体类名,column对应数据库列名-->
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <!--复杂的属性需要单独处理
            对象使用association
            集合用collection-->
        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
        </association>
    </resultMap>
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方式二、子查询

<!--    子查询-->
    <select id="getStudents" resultMap="Student2">
        select  * from student
    </select>
    
    <resultMap id="Student2" type="Student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <!--复杂的属性需要单独处理
            对象使用association
            集合用collection-->
<!--        property为对应实体类名,javaType返回值类型，getTeacher为子查询绑定的id-->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
    </resultMap>
    
    <select id="getTeacher" resultType="Teacher">
        select name from teacher where id=#{id}
    </select>
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6、创建测试类测试

package dao;

import org.apache.ibatis.session.SqlSession;
import pojo.Student;
import until.MyBatisUntil;

import java.util.List;

public class Test {
    @org.junit.Test
    public void test(){
        SqlSession sqlSession = MyBatisUntil.getSqlSession();

        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);

        List<Student> students = mapper.getStudents();

        for (Student student : students) {
            System.out.println(student);
        }

        sqlSession.close();
    }
}
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查询成功

 

原文地址：https://blog.csdn.net/weixin_50616848/article/details/122069955