Mybatis入门(六)联查之多对一

Mysql可以联查，但Mybatis也可以联查只是没有MySQL联查的舒服需要配置很多文件。

开始搭建环境：

MySQL新建两个表一个Student一个Teacher表：

Teacher表：

CREATE TABLE teacher(
 tid int primary key,
 tname varchar(30) not null
)

Student表：

CREATE TABLE student(
 sid int primary key,
 sname varchar(30) not null,
 tid int not null,
 foreign key(tid) references teacher(tid)
)

目录：

文件很多但也不复杂静下心来慢慢想就能捋清楚。

pom.xml文件配置:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>MybatisTest04</artifactId>
    <version>1.0-SNAPSHOT</version>
    <dependencies>
        <!--导入MySQL包/核心-->
        <!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.18</version>
        </dependency>

        <!--导入Mybatis包/核心-->
        <!-- https://mvnrepository.com/artifact/org.mybatis/mybatis -->
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.3</version>
        </dependency>

        <!--导入junit测试包-->
        <!-- https://mvnrepository.com/artifact/junit/junit -->
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.13</version>
            <scope>test</scope>
        </dependency>
        <!--偷懒专用包-->
        <!-- https://mvnrepository.com/artifact/org.projectlombok/lombok -->
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.10</version>
            <scope>provided</scope>
        </dependency>

    </dependencies>

    <!--maven由于他的约定大于配置，我们之后可以能遇到我们写的配置文件，无法被处理或生效问题，解决方案-->
    <build>
        <resources>
            <resource>
                <directory>src/main/java</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>false</filtering>
            </resource>
            <resource>
                <directory>src/main/resources</directory>
                <includes>
                    <include>**/*.properties</include>
                    <include>**/*.xml</include>
                </includes>
                <filtering>false</filtering>
            </resource>
        </resources>
    </build>

</project>

Mybatis-config.xml配置文件：

<?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>
    <properties resource="db.properties"/>

    <typeAliases>
        <typeAlias type="com.hdlf.pojo.student" alias="student"></typeAlias>
        <typeAlias type="com.hdlf.pojo.teacher" alias="teacher"></typeAlias>
    </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="${username}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <mapper resource="com/hdlf/dao/StudnetMapper.xml"></mapper>
        <mapper class="com.hdlf.dao.TeacherMapper"></mapper>
    </mappers>
</configuration>

utils工具类：

package com.hdlf.utlis;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.IOException;
import java.io.InputStream;

public class MybatisUtlis {

    private static SqlSessionFactory sqlSessionFactory;
    static{
        try {
            String resource = "Mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession();
    }
}

StudentMapper接口：

package com.hdlf.dao;

import com.hdlf.pojo.student;

import java.util.List;

public interface StudentMapper {
    student getStudent(int tid);

    List<student> getallstudent();
}

TeacherMapper接口:

package com.hdlf.dao;

import com.hdlf.pojo.teacher;

public interface TeacherMapper {
    teacher getteacher(int tid);
}

student实体类：

package com.hdlf.pojo;

public class student {
    private int sid;
    private String sname;
    private teacher teacher;

    public student() {
    }

    public student(int sid, String sname, com.hdlf.pojo.teacher teacher) {
        this.sid = sid;
        this.sname = sname;
        this.teacher = teacher;
    }

    public int getSid() {
        return sid;
    }

    public void setSid(int sid) {
        this.sid = sid;
    }

    public String getSname() {
        return sname;
    }

    public void setSname(String sname) {
        this.sname = sname;
    }

    public com.hdlf.pojo.teacher getTeacher() {
        return teacher;
    }

    public void setTeacher(com.hdlf.pojo.teacher teacher) {
        this.teacher = teacher;
    }

    @Override
    public String toString() {
        return "student{" +
                "sid=" + sid +
                ", sname='" + sname + '\'' +
                ", teacher=" + teacher +
                '}';
    }
}

teacher实体类:

package com.hdlf.pojo;

public class teacher {
    private int tid;
    private String tname;

    public teacher() {
    }

    public teacher(int tid, String tname) {
        this.tid = tid;
        this.tname = tname;
    }

    public int getTid() {
        return tid;
    }

    public void setTid(int tid) {
        this.tid = tid;
    }

    public String getTname() {
        return tname;
    }

    public void setTname(String tname) {
        this.tname = tname;
    }

    @Override
    public String toString() {
        return "teacher{" +
                "tid=" + tid +
                ", tname='" + tname + '\'' +
                '}';
    }
}

搭建完环境就该操作了，接下来很重要：

StudnetMapper.xml配置文件：

<association>装配对象时使用，其中 property 表示类中的属性名 select 表示要执行的sql语句(写完整的sql语句) column 要传过去的字段参数   javaType指定这个实体类

<?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.hdlf.dao.StudentMapper">

    <select id="getStudent" parameterType="int">
        select * from mybatis.student where sid = #{tid}
    </select>

    <select id="getallstudent" resultMap="getallstudentmap">
        select * from mybatis.student
    </select>

    <resultMap id="getallstudentmap" type="student">
        <result property="sid" column="sid"></result>
        <result property="sname" column="sname"></result>
        <!--<association>装配对象时使用，其中  property  表示类中的属性名
          select  表示要执行的sql语句(写完整的sql语句) column 要传过去的字段参数
           javaType指定这个实体类-->
        <association property="teacher" column="tid" javaType="teacher" select="getteacher"></association>
    </resultMap>

    <select id="getteacher" resultType="teacher">
        select * from mybatis.teacher where tid = #{tid}
    </select>
</mapper>

测试类：

MyTest：

public class MyTest {
    @Test
    public void test(){
        SqlSession sqlSession = MybatisUtlis.getSqlSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);

        List<student> s = mapper.getallstudent();
        for (student s2 : s){
            System.out.println(s2);
        }
        sqlSession.close();

    }
}

结果：

下一章说一对多的处理