Multi-table operation query one to one

1. One-to-one relationship

One-to-one relationship in the database-primary key association and foreign key key
such as person table and ID card table

1. Primary key association

create table person (
	pid int primary key auto_increment,
	pname varchar(40) not null,
	pgender varchar(10) not null
)

create table card (
	cid int not null, -- 引用person表的主键（由程序控制）
	cnumber varchar(18) not null
)

2. The foreign key association
person table remains unchanged, and the foreign key column of the card table is added

create table card (
	cid int not null primary key auto_increment,
	cnumber varchar(18) not null,
	pid int,
	foreign key(pid) references person(pid)) -- person的主键（应该程序控制）
)

2. One-to-one foreign key association query configuration

• sql Take user table and order table as examples, order and user one-to-one

create table `users1` (
	`userId` int (11),
	`userName` varchar (60),
	`userPassword` varchar (120),
	`birthday` bigint (255)
); 

create table `orders` (
	`order_id` int (11),
	`order_time` datetime ,
	`order_money` Decimal (11),
	`uid` int (11)
); 

package cn.bitqian.entity;

import java.sql.Timestamp;

/**
 * 用户订单表  一对一
 * 一个订单对应一个用户...
 * @author echo lovely
 * @date 2020/9/13 10:26
 */
public class Order {
    
    

    private Integer orderId;
    private Timestamp orderTime;
    private Double orderMoney;
	
    // 当前订单对应的用户..  面向对象表示
    private User user;
	// constructor.. set/get toString..
}

<?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="cn.bitqian.dao.OrderMapper">

    <!-- 给Order封装数据 -->
    <resultMap id="orderMap" type="Order">

        <id property="orderId" column="order_id"/>
        <result property="orderTime" column="order_time"/>
        <result property="orderMoney" column="order_money"/>
        <!--  封装order里面的user -->
        <!--<result property="user.userId" column="userid"/>
        <result property="user.userName" column="username"/>
        <result property="user.userPassword" column="userpassword"/>
        <result property="user.birthday" column="birthday"/>-->

        <!-- user属性 User类型-->
        <association property="user" javaType="User">
            <id property="userId" column="userid"/>
            <result property="userName" column="username" />
            <result property="userPassword" column="userpassword" />
            <result property="birthday" column="birthday" />
        </association>

    </resultMap>

    <!-- 返回的是order -->
    <select id="queryAllOrder" resultMap="orderMap">
        SELECT *, u.`userId` AS userid FROM orders o LEFT JOIN users1 u
        ON o.`uid` = u.`userId`
    </select>

</mapper>