一对一联表查询

一：

1.需求说明

　　根据班级id查询班级信息（带教师信息）

　　一个老师带一个班级。

　　

2.新建数据库

　　老师表

　　班级表

CREATE TABLE TEACHER(
    t_id INT PRIMARY KEY AUTO_INCREMENT,
    t_name VARCHAR(20)
);
CREATE TABLE CLASS(
    c_id INT PRIMARY KEY AUTO_INCREMENT,
    c_name VARCHAR(20),
    teacher_id INT
);
ALTER TABLE CLASS ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES TEACHER(t_id);
INSERT INTO teacher(T_NAME) VALUES('LS1');
INSERT INTO teacher(T_NAME) VALUES('LS1');
INSERT INTO CLASS(c_name,teacher_id) VALUES('bj1',1);
INSERT INTO CLASS(c_name,teacher_id) VALUES('bj2',2);

3.新建实体类--Teacher.java

package com.jun.bean;

public class Teacher {
    //
    private int id;
    private String name;
    //
    public Teacher() {}
    public Teacher(int id,String name) {
        this.id=id;
    }
    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;
    }
    @Override
    public String toString() {
        return "Teacher [id=" + id + ", name=" + name + "]";
    }
    
}

4.新建实体类--Classes.java（使用Class会出错）

package com.jun.bean;

public class Classes {
    //
    private int id;
    private String name;
    private Teacher teacher;
    //
    public Classes() {}
    public Classes(int id,String name,Teacher teacher) {
        this.id=id;
        this.name=name;
        this.teacher=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 getTeacher() {
        return teacher;
    }
    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }
    @Override
    public String toString() {
        return "Class [id=" + id + ", name=" + name + ", teacher=" + teacher + "]";
    }
    
}

二：联表查询

1.目录

　　

2.新建映射文件class.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="one2one">
    <!-- 
        方式一：
        select * from class c,teacher t where c.teacher_id=t.t_id and c.c_id=1 
     -->
     <select id="selectClass" parameterType="int" resultMap="getClass">
         select * from class c,teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
     </select>
     <resultMap type="com.jun.bean.Classes" id="getClass">
         <id property="id" column="c_id"/>
         <result property="name" column="c_name"/>
         <association property="teacher" javaType="com.jun.bean.Teacher">
             <id property="id" column="t_id"/>
             <result property="name" column="t_name"/>
         </association>
     </resultMap>
</mapper>

3.配置文件Configuration.xml

<?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>
  <environments default="development">
    <environment id="development">
      <transactionManager type="JDBC">
        <property name="" value=""/>
      </transactionManager>
      <dataSource type="UNPOOLED">
        <property name="driver" value="com.mysql.jdbc.Driver"/>
        <property name="url" value="jdbc:mysql://127.0.0.1:3308/mybatis"/>
        <property name="username" value="root"/>
        <property name="password" value="123456"/>
      </dataSource>
    </environment>
  </environments>

   <mappers>
      <mapper resource="com/jun/sql/config/class.xml"/>
   </mappers>

</configuration>

4.测试类

package com.jun.main;

import java.io.IOException;
import java.io.Reader;

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 org.junit.Test;
import com.jun.bean.Classes;
public class MainTest {
    @Test
    public void test1() throws Exception {
        Reader reader=Resources.getResourceAsReader("com/jun/config/Configuration.xml");
        SqlSessionFactory sqlSessionFactory=new SqlSessionFactoryBuilder().build(reader);
        SqlSession sqlSession=sqlSessionFactory.openSession(true);                //true后是自动提交
        String statement="one2one.selectClass";
        Classes classes=sqlSession.selectOne(statement, 1);
        System.out.println(classes);
        sqlSession.close();
    }
}

5.结果

　　

三：两次查询

1.目录结构

　　在原有的基础上继续增加方法

2.新建映射文件class.xml

　　在方式一的基础上增加了方式二。

　　方式二中需要的注意点是字段名与属性名不一致的解决方式，使用别名的处理方式。

<?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="one2one">
    <!-- 
        方式一：
        select * from class c,teacher t where c.teacher_id=t.t_id and c.c_id=1 
     -->
     <select id="selectClass" parameterType="int" resultMap="getClass">
         select * from class c,teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
     </select>
     <resultMap type="com.jun.bean.Classes" id="getClass">
         <id property="id" column="c_id"/>
         <result property="name" column="c_name"/>
         <association property="teacher" javaType="com.jun.bean.Teacher">
             <id property="id" column="t_id"/>
             <result property="name" column="t_name"/>
         </association>
     </resultMap>
     
     <!-- 
         方式二：
         select * from class where c_id=1;
         select * from teacher where t_id=1; //这个1是上面的语句查询出来的结果
      -->
      <select id="selectClass2" parameterType="int" resultMap="getClass2">
          select * from class where c_id=#{id}
      </select>
      <select id="getTeacher" parameterType="int" resultType="com.jun.bean.Teacher">
          select t_id id,t_name name from teacher where t_id=#{id}
      </select>
      <resultMap type="com.jun.bean.Classes" id="getClass2">
          <id property="id" column="c_id"/>
         <result property="name" column="c_name"/>
         <association property="teacher" column="teacher_id" select="getTeacher"></association>
      </resultMap>
</mapper>

3.配置文件--Configuration.xml

　　这个不用改变

4.测试类

package com.jun.main;

import java.io.IOException;
import java.io.Reader;

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 org.junit.Test;
import com.jun.bean.Classes;
public class MainTest {
    /**
     * 方式一
     * @throws Exception
     */
    @Test
    public void test1() throws Exception {
        Reader reader=Resources.getResourceAsReader("com/jun/config/Configuration.xml");
        SqlSessionFactory sqlSessionFactory=new SqlSessionFactoryBuilder().build(reader);
        SqlSession sqlSession=sqlSessionFactory.openSession(true);                //true后是自动提交
        String statement="one2one.selectClass";
        Classes classes=sqlSession.selectOne(statement, 1);
        System.out.println(classes);
        sqlSession.close();
    }
    /**
     * 方式二
     */
    @Test
    public void test2() throws Exception {
        Reader reader=Resources.getResourceAsReader("com/jun/config/Configuration.xml");
        SqlSessionFactory sqlSessionFactory=new SqlSessionFactoryBuilder().build(reader);
        SqlSession sqlSession=sqlSessionFactory.openSession(true);                //true后是自动提交
        String statement="one2one.selectClass2";
        Classes classes=sqlSession.selectOne(statement, 2);
        System.out.println(classes);
        sqlSession.close();
    }
}

5.测试结果