Mybatis学习——一对一关联表查询

1.SQL语句建表

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('LS2');
INSERT INTO class(c_name, teacher_id) VALUES('bj_a', 1); INSERT INTO class(c_name, teacher_id) VALUES('bj_b', 2);

2.实体类

package com.zhengbin.entity;

public class Teacher {
    private int id;
    private String name;
    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 + "]";
    }
    public Teacher(int id, String name) {
        super();
        this.id = id;
        this.name = name;
    }
    public Teacher() {
        super();
        // TODO Auto-generated constructor stub
    }
}

package com.zhengbin.entity;

public class Classes {
    private int id;
    private String name;
    private Teacher teacher;
    @Override
    public String toString() {
        return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher + "]";
    }
    public Classes() {
        super();
    }
    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;
    }
}

3.实体类映射文件

　　两种方式：

　　　　1. 联表查询

　　　　　　关键是返回值的设置

　　　　2. 执行两次查询

　　　　　　关键是返回值的设置，和第二次查询所需参数的传递

<?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">
<!-- 每个映射文件的namespace应该是唯一的 -->
<mapper namespace="com.zhengbin.entity.classMapper">
    <!-- parameterType 参数表示需要参数的类型 -->
    <!-- resultType 参数表示返回结果的类型，该可以写为实体包的全路径，或者在conf.xml配置文件中，声明实体的别名 -->
    
    <!-- 
        方式一：
            嵌套结果：使用嵌套结果映射来处理重复的联合结果的子集 封装联表查询的数据(去除重复的数据)
     -->
    <select id="getClass" parameterType="int" resultMap="getClassMap">
        select * from class c,teacher t where c.teacher_id=t.t_id and c.c_id=#{id};
    </select>
    <resultMap type="Classes" id="getClassMap">
        <id property="id" column="c_id"/>
        <result property="name" column="c_name"/>
        <association property="teacher" javaType="Teacher">
            <id property="id" column="t_id"/>
            <result property="name" column="t_name"/>
        </association>
    </resultMap>
    <!-- 
        方式二：
            嵌套查询：通过执行另外一个 SQL 映射语句来返回预期的复杂类型 
                    SELECT * FROM class WHERE c_id=1;
                    SELECT * FROM teacher WHERE t_id=1 //1 是上一个查询得到的 teacher_id 的值
     -->
     <select id="getClass2" parameterType="int" resultMap="getClass2Map">
         select * from class where c_id=#{id}
     </select>
     <select id="getTeacher" parameterType="int" resultType="Teacher">
         select t_id id,t_name name from teacher where t_id=#{id}
     </select>
     <resultMap type="Classes" id="getClass2Map">
         <id property="id" column="c_id"/>
         <result property="name" column="c_name"/>
         <association property="teacher" select="getTeacher" column="teacher_id"/>
     </resultMap>
</mapper>

4.测试类

package com.zhengbin.test;

import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;

import com.zhengbin.entity.Classes;
import com.zhengbin.util.MyBatisUtils;

public class Test3 {
    @org.junit.Test
    public void getTeacher(){
        SqlSessionFactory sessionFactory = MyBatisUtils.getFactory();
        // 参数为TRUE，相当于session.commit();
        SqlSession session = sessionFactory.openSession(true);
        // 读取映射文件
        String statement = "com.zhengbin.entity.classMapper" + ".getClass";
//        String statement = "com.zhengbin.entity.classMapper" + ".getClass2";
        Classes c = session.selectOne(statement,1);
        System.out.println(c);
        session.close();
    }
}

5.几个关键的属性

association : 用于一对一的关联查询
property : 对象属性的名称
javaType : 对象属性的类型
column : 所对应的外键字段名称
select : 使用另一个查询封装的结果