Mybatis入门（二）

于 2018-11-13 08:33:42 发布
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分类专栏： Java Mybatis 文章标签： Java Mybatis
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本文链接：https://blog.youkuaiyun.com/u010995220/article/details/84020465
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一、数据模型分析思路

1、每张表记录的内容

      分模块对每张表的内容进行熟悉，相当于学习系统需求（功能）的过程；

2、每张表的重要字段

      非空字段、外键字段

3、数据库级别表与表之间的外键关系

4、数据库级别表与表之间的业务关系

      一定是建立在业务意义的基础上；

      先分析数据库级别有关联的表间的业务关系；

      再分析数据库级别没有关联的表之间是否有业务关系；

二、商品订单数据模型

2.1、数据模型分析

2.2、一对一查询

2.2.1、需求

查询订单，并关联查询创建订单的用户信息

2.2.2、分析

查询的主表：订单表

查询的关联表：用户表

创建sql语句：

select 
orders.*,
user.username,
user.sex,
user.address 
from orders,user 
where orders.user_id=user.id

2.2.2、resultType实现

1、创建POJO

package com.bird.po;

import java.util.Date;

/**
 * @Author:wangqk
 * @Date:2018/11/14 上午7:31
 */
public class Orders {
    private Integer id;
    private Integer userId;
    private String number;
    private Date createTime;
    private String note;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public Integer getUserId() {
        return userId;
    }

    public void setUserId(Integer userId) {
        this.userId = userId;
    }

    public String getNumber() {
        return number;
    }

    public void setNumber(String number) {
        this.number = number;
    }

    public Date getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }

    public String getNote() {
        return note;
    }

    public void setNote(String note) {
        this.note = note;
    }
}

package com.bird.po;

/**
 * 映射订单和用户的查询结果，继承包含字段较多的pojo类
 *
 * @Author:wangqk
 * @Date:2018/11/14 上午7:36
 */
public class OrdersCustom extends Orders {
    private String userName;
    private String sex;
    private String address;

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }
}

2、创建mapper.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="com.bird.mapper.OrdersCustomerMapper">
    <!-- 查询订单，关联查询用户 -->
    <select id="findOrdersUser" resultType="com.bird.po.OrdersCustom">
        select
            orders.*,
            user.username,
            user.sex,
            user.address
        from orders, user
        where orders.user_id = user.id
    </select>

</mapper>

3、创建mapper.java

package com.bird.mapper;

import com.bird.po.OrdersCustom;

import java.util.List;

/**
 * @Author:wangqk
 * @Date:2018/11/14 上午7:53
 */
public interface OrdersCustomerMapper {
    /**
     * 查询订单，关联查询用户
     *
     * @return
     * @throws Exception
     */
    List<OrdersCustom> findOrdersUser() throws Exception;
}

4、单元测试

package com.bird.mapper;

import com.bird.po.OrdersCustom;
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.Before;
import org.junit.Test;

import java.io.InputStream;
import java.util.List;

/**
 * @Author:wangqk
 * @Date:2018/11/14 上午7:57
 */
public class OrdersCustomMapperTest {

    private SqlSessionFactory sqlSessionFactory;

    @Before
    public void setUp() throws Exception {
        // mybatis配置文件
        String resource = "SqlMapConfig.xml";
        // 得到配置文件流
        InputStream inputStream = Resources.getResourceAsStream(resource);
        // 创建会话工厂
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    }

    @Test
    public void findOrdersUserTest() throws Exception {
        SqlSession sqlSession = sqlSessionFactory.openSession();
        // 创建代理对象
        OrdersCustomMapper ordersCustomMapper = sqlSession.getMapper(OrdersCustomMapper.class);
        // 调用mapper方法
        List<OrdersCustom> list = ordersCustomMapper.findOrdersUser();
        System.out.println(list);
        sqlSession.close();
    }
}

2.2.3、resultMap实现

使用resultMap将查询结果中的订单信息映射到Orders对象。

在Orders类中添加user属性，并将关联查询的用户信息映射到Orders对象的user属性中。

1、创建POJO（User类、Orders类）

package com.bird.po;

import java.util.Date;

/**
 * @Author:wangqk
 * @Date:2018/11/14 上午7:30
 */
public class User {
    // 属性名和数据库表的字段名对应
    private int id;
    private String userName;
    private String sex;
    private Date birthday;
    private String address;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public Date getBirthday() {
        return birthday;
    }

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }
}

package com.bird.po;

import java.util.Date;

/**
 * @Author:wangqk
 * @Date:2018/11/14 上午7:31
 */
public class Orders {
    private Integer id;
    private Integer userId;
    private String number;
    private Date createTime;
    private String note;

    private User user;

    public User getUser() {
        return user;
    }

    public void setUser(User user) {
        this.user = user;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public Integer getUserId() {
        return userId;
    }

    public void setUserId(Integer userId) {
        this.userId = userId;
    }

    public String getNumber() {
        return number;
    }

    public void setNumber(String number) {
        this.number = number;
    }

    public Date getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }

    public String getNote() {
        return note;
    }

    public void setNote(String note) {
        this.note = note;
    }
}

2、创建mapper.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="com.bird.mapper.OrdersCustomMapper">

    <!-- 查询订单关联用户：将查询结果集映射到com.bird.po.Orders中 -->
    <resultMap id="OrdersUserResultMap" type="com.bird.po.Orders">
        <!-- 配置映射的订单信息 -->
        <!-- id指定查询字段中的唯一标识，如果有多个字段组成唯一标识，则配置多个id -->
        <!-- column为订单信息的唯一标识字段，property为订单信息唯一标识字段所映射到Orders中的属性名 -->
        <id column="id" property="id"/>
        <result column="user_id" property="userId"/>
        <result column="number" property="number"/>
        <result column="createtime" property="createTime"/>
        <result column="note" property="note"/>

        <!-- 配置映射的关联用户信息 -->
        <!-- association关联查询单个对象的信息，property关联查询字段映射到Orders中的属性，javaType为映射到用户的属性名 -->
        <association property="user" javaType="com.bird.po.User">
            <!-- id为关联查询用户的唯一标识，column指定用户信息的唯一标识字段 -->
            <id column="user_id" property="id"/>
            <result column="username" property="userName"/>
            <result column="sex" property="sex"/>
            <result column="address" property="address"/>
        </association>
    </resultMap>
    <select id="findOrdersUser" resultMap="OrdersUserResultMap">
        select
            orders.*,
            user.username,
            user.sex,
            user.address
        from orders, user
        where orders.user_id = user.id
    </select>

</mapper>

3、创建mapper.java

package com.bird.mapper;

import com.bird.po.Orders;

import java.util.List;

/**
 * @Author:wangqk
 * @Date:2018/11/14 上午7:53
 */
public interface OrdersCustomMapper {
    /**
     * 查询订单，关联查询用户
     *
     * @return
     * @throws Exception
     */
    List<Orders> findOrdersUser() throws Exception;
}

4、单元测试同上。

2.2.4、总结resultType和resultMap实现一对一查询

resultType实现较为简单，如果pojo中没有包含查询字段，则要增加字段对应的属性名才能完成映射。如果没有查询结果的特殊要求，建议使用resultType。

resultMap实现较为复杂，需要单独定义resultMap。如果对查询结果有特殊的要求，使用resultMap可以完成将关联查询的结果集映射到pojo的属性中。

补充：resultMap可以实现延迟加载。

2.3、一对多查询

2.3.1、需求

查询订单及订单明细。

2.3.2、分析

查询的主表：订单表

查询的关联表：订单明细表

创建sql语句：

select
            orders.*,
            user.username,
            user.sex,
            user.address,
            orderdetail.id as orderdetail_id,
            orderdetail.items_id,
            orderdetail.items_num,
            orderdetail.orders_id
        from
            orders,
            user,
            orderdetail
        where orders.user_id = user.id and orderdetail.orders_id = orders.id

2.3.3、 resultMap实现

1、创建POJO

package com.bird.po;

import java.util.Date;
import java.util.List;

/**
 * @Author:wangqk
 * @Date:2018/11/14 上午7:31
 */
public class Orders {
    private Integer id;
    private Integer userId;
    private String number;
    private Date createTime;
    private String note;
    // 用户信息
    private User user;

    // 订单明细
    private List<OrderDetail> orderDetails;

    public List<OrderDetail> getOrderDetails() {
        return orderDetails;
    }

    public void setOrderDetails(List<OrderDetail> orderDetails) {
        this.orderDetails = orderDetails;
    }

    public User getUser() {
        return user;
    }

    public void setUser(User user) {
        this.user = user;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public Integer getUserId() {
        return userId;
    }

    public void setUserId(Integer userId) {
        this.userId = userId;
    }

    public String getNumber() {
        return number;
    }

    public void setNumber(String number) {
        this.number = number;
    }

    public Date getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }

    public String getNote() {
        return note;
    }

    public void setNote(String note) {
        this.note = note;
    }
}

2、创建mapper.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="com.bird.mapper.OrdersCustomMapper">
   
    <!-- 查询订单关联用户：将查询结果集映射到com.bird.po.Orders中 -->
    <resultMap id="OrdersUserResultMap" type="com.bird.po.Orders">
        <!-- 配置映射的订单信息 -->
        <!-- id指定查询字段中的唯一标识，如果有多个字段组成唯一标识，则配置多个id -->
        <!-- column为订单信息的唯一标识字段，property为订单信息唯一标识字段所映射到Orders中的属性名 -->
        <id column="id" property="id"/>
        <result column="user_id" property="userId"/>
        <result column="number" property="number"/>
        <result column="createtime" property="createTime"/>
        <result column="note" property="note"/>

        <!-- 配置映射的关联用户信息 -->
        <!-- association关联查询单个对象的信息，property关联查询字段映射到Orders中的属性 -->
        <association property="user" javaType="com.bird.po.User">
            <!-- id为关联查询用户的唯一标识，column指定用户信息的唯一标识字段，javaType为映射到用户的属性名 -->
            <id column="user_id" property="id"/>
            <result column="username" property="userName"/>
            <result column="sex" property="sex"/>
            <result column="address" property="address"/>
        </association>
    </resultMap>
    <select id="findOrdersUser" resultMap="OrdersUserResultMap">
        select
            orders.*,
            user.username,
            user.sex,
            user.address
        from orders, user
        where orders.user_id = user.id
    </select>

    <!-- 查询订单关联的用户及订单明细 -->
    <!-- 使用extends继承，无需配置订单和用户信息的映射 -->
    <resultMap id="OrdersAndOrderDetails" type="com.bird.po.Orders" extends="OrdersUserResultMap">
        <!-- 订单信息 -->
        <!-- 用户信息 -->

        <!-- 订单明细信息 -->
        <!-- 一个订单关联查询多条明细，使用collection进行一对多的映射 -->
        <!--
        collection将关联查询到的多条记录映射到集合对象
        property将关联查询到的多条记录映射到属性名
        ofType指定集合属性的pojo类型
         -->
        <collection property="orderDetails" ofType="com.bird.po.OrderDetail">
            <!-- id为订单明细的唯一标识，property字段映射的属性名 -->
            <id property="id" column="orderdetail_id"/>
            <result column="items_id" property="itemsId"/>
            <result column="items_num" property="itemsNum"/>
            <result column="orders_id" property="ordersId"/>
        </collection>
    </resultMap>
    <select id="findOrdersAndOrderDetails" resultMap="OrdersAndOrderDetails">
        select
            orders.*,
            user.username,
            user.sex,
            user.address,
            orderdetail.id as orderdetail_id,
            orderdetail.items_id,
            orderdetail.items_num,
            orderdetail.orders_id
        from
            orders,
            user,
            orderdetail
        where orders.user_id = user.id and orderdetail.orders_id = orders.id
    </select>

</mapper>

3、创建mapper.java

package com.bird.mapper;

import com.bird.po.Orders;

import java.util.List;

/**
 * @Author:wangqk
 * @Date:2018/11/14 上午7:53
 */
public interface OrdersCustomMapper {
    /**
     * 查询订单，关联查询用户
     *
     * @return
     * @throws Exception
     */
    List<Orders> findOrdersUser() throws Exception;

    /**
     * 查询订单（关联用户）及订单明细
     *
     * @return
     * @throws Exception
     */
    List<Orders> findOrdersAndOrderDetails() throws Exception;
}

4、单元测试

package com.bird.mapper;

import com.bird.po.Orders;
import com.bird.po.OrdersCustom;
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.Before;
import org.junit.Test;

import java.io.InputStream;
import java.util.List;

/**
 * @Author:wangqk
 * @Date:2018/11/14 上午7:57
 */
public class OrdersCustomMapperTest {

    private SqlSessionFactory sqlSessionFactory;

    @Before
    public void setUp() throws Exception {
        // mybatis配置文件
        String resource = "SqlMapConfig.xml";
        // 得到配置文件流
        InputStream inputStream = Resources.getResourceAsStream(resource);
        // 创建会话工厂
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    }

    @Test
    public void findOrdersUserTest() throws Exception {
        SqlSession sqlSession = sqlSessionFactory.openSession();
        // 创建代理对象
        OrdersCustomMapper ordersCustomMapper = sqlSession.getMapper(OrdersCustomMapper.class);
        // 调用mapper方法
        List<Orders> list = ordersCustomMapper.findOrdersUser();
        System.out.println(list);
        sqlSession.close();
    }

    @Test
    public void findOrdersAndOrderDetails() throws Exception {
        SqlSession sqlSession = sqlSessionFactory.openSession();
        // 创建代理对象
        OrdersCustomMapper ordersCustomMapper = sqlSession.getMapper(OrdersCustomMapper.class);
        // 调用mapper方法
        List<Orders> list = ordersCustomMapper.findOrdersAndOrderDetails();
        System.out.println(list);
        sqlSession.close();
    }
}

2.4、多对多查询

2.4.1、需求

查询用户及购买商品信息

2.4.2、分析

查询的主表：用户表

查询的关联表：订单表、订单明细表、商品表

创建sql：

SELECT
            orders.*,
            USER.username,
            USER.sex,
            USER.address,
            orderdetail.id orderdetail_id,
            orderdetail.items_id,
            orderdetail.items_num,
            orderdetail.orders_id,
            items.name     items_name,
            items.detail   items_detail,
            items.price    items_price
        FROM
            orders,
            USER,
            orderdetail,
            items
        WHERE orders.user_id = user.id AND orderdetail.orders_id = orders.id AND orderdetail.items_id = items.id

2.4.3、resultMap实现

1、创建POJO

package com.bird.po;

import java.util.Date;
import java.util.List;

/**
 * @Author:wangqk
 * @Date:2018/11/14 上午7:30
 */
public class User {
    // 属性名和数据库表的字段名对应
    private int id;
    private String userName;
    private String sex;
    private Date birthday;
    private String address;

    public List<Orders> getOrdersList() {
        return ordersList;
    }

    public void setOrdersList(List<Orders> ordersList) {
        this.ordersList = ordersList;
    }

    private List<Orders> ordersList;


    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public Date getBirthday() {
        return birthday;
    }

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }
}

package com.bird.po;

import java.util.Date;
import java.util.List;

/**
 * @Author:wangqk
 * @Date:2018/11/14 上午7:31
 */
public class Orders {
    private Integer id;
    private Integer userId;
    private String number;
    private Date createTime;
    private String note;
    // 用户信息
    private User user;

    // 订单明细
    private List<OrderDetail> orderDetails;

    public List<OrderDetail> getOrderDetails() {
        return orderDetails;
    }

    public void setOrderDetails(List<OrderDetail> orderDetails) {
        this.orderDetails = orderDetails;
    }

    public User getUser() {
        return user;
    }

    public void setUser(User user) {
        this.user = user;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public Integer getUserId() {
        return userId;
    }

    public void setUserId(Integer userId) {
        this.userId = userId;
    }

    public String getNumber() {
        return number;
    }

    public void setNumber(String number) {
        this.number = number;
    }

    public Date getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }

    public String getNote() {
        return note;
    }

    public void setNote(String note) {
        this.note = note;
    }
}

package com.bird.po;

/**
 * @Author:wangqk
 * @Date:2018/11/14 上午7:32
 */
public class OrderDetail {
    private Integer id;
    private Integer ordersId;
    private Integer itemsId;
    private Integer itemsNum;
    // 明细对应的商品信息
    private Items items;

    public Items getItems() {
        return items;
    }

    public void setItems(Items items) {
        this.items = items;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public Integer getOrdersId() {
        return ordersId;
    }

    public void setOrdersId(Integer ordersId) {
        this.ordersId = ordersId;
    }

    public Integer getItemsId() {
        return itemsId;
    }

    public void setItemsId(Integer itemsId) {
        this.itemsId = itemsId;
    }

    public Integer getItemsNum() {
        return itemsNum;
    }

    public void setItemsNum(Integer itemsNum) {
        this.itemsNum = itemsNum;
    }
}

package com.bird.po;

import java.util.Date;

/**
 * @Author:wangqk
 * @Date:2018/11/26 下午10:54
 */
public class Items {
    private Integer id;
    private String name;
    private Float price;
    private String pic;
    private Date createTime;
    private String detail;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Float getPrice() {
        return price;
    }

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

    public String getPic() {
        return pic;
    }

    public void setPic(String pic) {
        this.pic = pic;
    }

    public Date getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }

    public String getDetail() {
        return detail;
    }

    public void setDetail(String detail) {
        this.detail = detail;
    }
}

2、创建mapper.xml

<!-- 查询用户及购买的商品 -->
    <resultMap id="UserAndItems" type="com.bird.po.User">
        <!-- 用户信息 -->
        <id property="id" column="user_id"/>
        <result column="username" property="userName"/>
        <result column="sex" property="sex"/>
        <result column="address" property="address"/>

        <!-- 订单信息 -->
        <!-- 一个用户对应多个订单，使用collection映射 -->
        <collection property="ordersList" ofType="com.bird.po.Orders">
            <id column="id" property="id"/>
            <result column="user_id" property="userId"/>
            <result column="number" property="number"/>
            <result column="create_time" property="createTime"/>
            <result column="note" property="note"/>

            <!-- 订单明细 -->
            <!-- 一个订单包含多个明细 -->
            <collection property="orderDetails" ofType="com.bird.po.OrderDetail">
                <id column="orderdetail_id" property="id"/>
                <result column="items_id" property="itemsId"/>
                <result column="items_num" property="itemsNum"/>
                <result column="orders_id" property="ordersId"/>

                <!-- 商品信息 -->
                <!-- 一个订单明细对应一个商品 -->
                <association property="items" javaType="com.bird.po.Items">
                    <id column="items_id" property="id"/>
                    <result column="items_name" property="name"/>
                    <result column="items_detail" property="detail"/>
                    <result column="items_price" property="price"/>
                </association>
            </collection>
        </collection>
    </resultMap>
    <select id="findUserAndItems" resultMap="UserAndItems">
        SELECT
            orders.*,
            USER.username,
            USER.sex,
            USER.address,
            orderdetail.id orderdetail_id,
            orderdetail.items_id,
            orderdetail.items_num,
            orderdetail.orders_id,
            items.name     items_name,
            items.detail   items_detail,
            items.price    items_price
        FROM
            orders,
            USER,
            orderdetail,
            items
        WHERE orders.user_id = user.id AND orderdetail.orders_id = orders.id AND orderdetail.items_id = items.id

    </select>

3、创建mapper.java

package com.bird.mapper;

import com.bird.po.Orders;
import com.bird.po.User;

import java.util.List;

/**
 * @Author:wangqk
 * @Date:2018/11/14 上午7:53
 */
public interface OrdersCustomMapper {

    List<User> findUserAndItems() throws Exception;
}

4、单元测试

package com.bird.mapper;

import com.bird.po.Orders;
import com.bird.po.OrdersCustom;
import com.bird.po.User;
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.Before;
import org.junit.Test;

import java.io.InputStream;
import java.util.List;

/**
 * @Author:wangqk
 * @Date:2018/11/14 上午7:57
 */
public class OrdersCustomMapperTest {

    private SqlSessionFactory sqlSessionFactory;

    @Before
    public void setUp() throws Exception {
        // mybatis配置文件
        String resource = "SqlMapConfig.xml";
        // 得到配置文件流
        InputStream inputStream = Resources.getResourceAsStream(resource);
        // 创建会话工厂
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    }

    @Test
    public void findUserAndItemsTest() throws Exception {
        SqlSession sqlSession = sqlSessionFactory.openSession();
        // 创建代理对象
        OrdersCustomMapper ordersCustomMapper = sqlSession.getMapper(OrdersCustomMapper.class);
        // 调用mapper方法
        List<User> list = ordersCustomMapper.findUserAndItems();
        System.out.println(list);
        sqlSession.close();
    }
}

2.5、总结

resultType：将查询结果按照sql列名pojo属性名一致性映射到pojo中。

resultMap：使用association和collection完成一对一和一对多高级映射（对结果有特殊的映射要求）。

association：将关联查询信息映射到一个pojo对象中。

collection：将关联查询信息映射到一个list集合中。

三、延迟加载

3.1、什么是延迟加载

延迟加载：先从单表查询、需要时再从关联表去关联查询，大大提高 数据库性能，因为查询单表要比关联查询多张表速度快。

3.2、使用association实现延迟加载

3.2.1、需求

查询订单且关联查询用户信息

 

 

 

 

 

 

 

 

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