5、Mybatis实现一对一关联表查询

本章节主要实现两个一对一关联表的查询，例如表PERSON



表PHONE



上述两个表一对一关联，一个人只能有一部手机。例如查询id为1的那个人的手机价格，sql 查询语句为：

SELECT * FROM person p1, phone p2 WHERE p1.phone_brand = p2.phone_brand AND p1.id=1

下面通过Mybaits实现此一对一关联表查询，主要通过两种方式来实现：封装查询和嵌套查询

一、封装查询

封装查询把两个表格封装在一条sql语句中进行查询

SELECT * FROM person p1, phone p2 WHERE p1.phone_brand = p2.phone_brand AND p1.id=1

1、首先创建表对应的实体类Person和Phone

Person类：

package com.lzj.mybaits.test1;

public class Person {

private int id;
private String name;
private Phone phone;

/*一定要加一个空的初始化方法，否则mybaits会报错没有空的初始化方法*/
public Person() {
super();
}

public Person(int id, String name, Phone phone) {
super();
this.id = id;
this.name = name;
this.phone = phone;
}

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 Phone getPhone() {
return phone;
}

public void setPhone(Phone phone) {
this.phone = phone;
}

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

}

Phone类：

package com.lzj.mybaits.test1;

public class Phone {

private String brand;
private float price;

/*一定要加一个无参构造函数*/
public Phone() {
super();
}

public Phone(String brand, float price) {
super();
this.brand = brand;
this.price = price;
}

public String getBrand() {
return brand;
}

public void setBrand(String brand) {
this.brand = brand;
}

public float getPrice() {
return price;
}

public void setPrice(float price) {
this.price = price;
}

@Override
public String toString() {
return "Phone [brand=" + brand + ", price=" + price + "]";
}

}

注意：创建的对应表的两个实体类一定要有无参的构造函数，mybaits时默认用无参的构造函数进行初始化对象。

2、定义sql 的映射文件personMapper.xml



3、定义mybaits的配置文件conf.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>

<typeAliases>
<package name="com.lzj.mybaits.test1" />
</typeAliases>

<environments default="development">
<environment id="development">
<transactionManager type="JDBC" />
<dataSource type="POOLED">
<!-- 配置数据源 -->
<property name="driver" value="com.mysql.jdbc.Driver" />
<property name="url" value="jdbc:mysql://localhost:3306/lzj_database" />
<property name="username" value="root" />
<property name="password" value="lzjlzj" />
</dataSource>
</environment>
</environments>

<!-- 指定sql的映射文件，如果有多个，都可以加进来 -->
<mappers>
<mapper resource="com/lzj/mybaits/test1/personMapper.xml" />
</mappers>

</configuration>

4、创建测试类

package com.lzj.mybaits.test1;
import java.io.InputStream;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

public class MybaitsTest {

public static void main(String[] args) throws Exception {
String resource = "conf.xml";
InputStream in = MybaitsTest.class.getClassLoader().getResourceAsStream(resource);
SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);
/*自动提交事务*/
SqlSession session = factory.openSession(true);
String statement = "com.lzj.mybaits.test1.userMapper.getPerson";
Person person = session.selectOne(statement, 1);
System.out.println(person);
session.close();
}

}

运行测试类，输出结果

Person [id=0, name=null, phone=Phone [brand=iphone, price=6000.0]]

二、嵌套查询

嵌套查询就是先查询一个表格，然后在第一次查询结果中再进行第二个表格查询。还是以上面的sql语句为例

select * from PERSON where id=1;//查询phone_brand为iphone
select * from PHONE where phone_brand='iphone'

只需要把personMapper.xml配置文件改成如下形式



小调用getPerson2对应的sql语句，把查询到的phone_brand在传入到getPhone继续调用第二条sql语句。运行测试类，输出结果同上。