尚硅谷-mybatis-(一对一关联)

提出需求：

根据班级id查询班级信息(带老师的信息)

项目结构：

创建表和数据：

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);

Teacher实体类：

package com.atguigu.mybatis.bean;

public class Teacher {

	private int id;
	private String name;

	public Teacher(int id, String name) {
		super();
		this.id = id;
		this.name = name;
	}

	public Teacher() {
		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;
	}

	@Override
	public String toString() {
		return "Teacher [id=" + id + ", name=" + name + "]";
	}

}

Classes实体类：

package com.atguigu.mybatis.bean;

public class Classes {

	private int id;
	private String name;
	private Teacher teacher;

	public Classes(int id, String name, Teacher teacher) {
		super();
		this.id = id;
		this.name = name;
		this.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;
	}

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

}

classesMapper.xml映射文件代码：

<?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="com.atguigu.mybatis.bean.classesMapper">

	<!--
		根据班级id查询班级信息(带老师的信息)
		##1. 联表查询
		SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=1;

		##2. 执行两次查询
		SELECT * FROM class WHERE c_id=1;  //teacher_id=1
		SELECT * FROM teacher WHERE t_id=1;//使用上面得到的teacher_id
	 -->
	<!--
		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 * from class c, teacher t where c.teacher_id=t.t_id and  c.c_id=1
	-->
	<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" column="teacher_id" select="getTeacher">
		</association>
	</resultMap>

</mapper>

获取SqlSessionFactory工厂的MybatisUtils代码：

package com.atguigu.mybatis.utils;

import java.io.InputStream;

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

public class MybatisUtils {

	public static SqlSessionFactory getFactory() {
		String resource = "conf.xml";
		InputStream inputStream = MybatisUtils.class.getClassLoader().getResourceAsStream(resource);

		SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);

		return factory;
	}
}

配置文件conf.xml代码：

<?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"/>
	<!-- 为实体类定义别名,简化sql映射xml文件中的引用-->
	<typeAliases>
		<!-- 该包下的所有类以其简单类名为实体类别名（如：User类的别名为User） -->
		<package name="com.atguigu.mybatis.bean"/>
	</typeAliases>
	<!--
		development : 开发模式
		work : 工作模式
 	-->
	<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="${name}" />
				<property name="password" value="${password}" />
			</dataSource>
		</environment>
	</environments>

	<mappers>
		<mapper resource="com/atguigu/mybatis/bean/classesMapper.xml"/>
	</mappers>
</configuration>

测试类Test4代码：

package com.atguigu.mybatis.test4;

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

import com.atguigu.mybatis.bean.Classes;
import com.atguigu.mybatis.utils.MybatisUtils;

/*
 * 测试: 一对一关联表查询
 */
public class Test4 {

	public static void main(String[] args) {

		SqlSessionFactory factory=MybatisUtils.getFactory();
		SqlSession session=factory.openSession();

		String statement="com.atguigu.mybatis.bean.classesMapper.getClass";
		statement="com.atguigu.mybatis.bean.classesMapper.getClass2";

		Classes c =session.selectOne(statement, 2);
		System.out.println(c);

		session.close();
	}
}