（七）MyBatis從入門到入土——花式查詢-有解無憂

這是mybatis系列第7篇，沒看前文的建議先去【Java冢狐】公眾號中查看前文，方便理解和掌握，上一篇中我們講述了一些關于獲取主鍵的方法，這篇文章中我們將會更加深入的介紹一些查詢的方法，特別是多表查詢，

在開始之前先開始諸如建庫建表等準備作業，

建庫建表

創建一個db：mybatisdemo

4張表:

user(用戶表)
goods(商品表)
orders(訂單表)
order_detail(訂單明細表)

表之間的關系：

orders和user是一對一的關系，一條訂單關聯一個用戶記錄
orders和order_detail是一對多關系，每個訂單中可能包含多個子訂單，每個子訂單對應一個商品

具體的建表陳述句如下所示：

DROP DATABASE IF EXISTS `mybatisdemo`;
CREATE DATABASE `mybatisdemo`;
USE `mybatisdemo`;
DROP TABLE IF EXISTS user;
CREATE TABLE user(
  id int AUTO_INCREMENT PRIMARY KEY COMMENT '用戶id',
  name VARCHAR(32) NOT NULL DEFAULT '' COMMENT '用戶名'
) COMMENT '用戶表';
INSERT INTO user VALUES (1,'冢狐'),(2,'Java冢狐');
DROP TABLE IF EXISTS goods;
CREATE TABLE goods(
  id int AUTO_INCREMENT PRIMARY KEY COMMENT '商品id',
  name VARCHAR(32) NOT NULL DEFAULT '' COMMENT '商品名稱',
  price DECIMAL(10,2) NOT NULL DEFAULT 0 COMMENT '商品價格'
) COMMENT '商品資訊表';
INSERT INTO goods VALUES (1,'Mybatis系列',8.00),(2,'spring系列',16.00);
DROP TABLE IF EXISTS orders;
CREATE TABLE orders(
  id int AUTO_INCREMENT PRIMARY KEY COMMENT '訂單id',
  user_id INT NOT NULL DEFAULT 0 COMMENT '用戶id，來源于user.id',
  create_time BIGINT NOT NULL DEFAULT 0 COMMENT '訂單創建時間(時間戳，秒)',
  up_time BIGINT NOT NULL DEFAULT 0 COMMENT '訂單最后修改時間(時間戳，秒)'
) COMMENT '訂單表';
INSERT INTO orders VALUES (1,2,unix_timestamp(now()),unix_timestamp(now())),(2,1,unix_timestamp(now()),unix_timestamp(now()));
DROP TABLE IF EXISTS order_detail;
CREATE TABLE order_detail(
  id int AUTO_INCREMENT PRIMARY KEY COMMENT '訂單明細id',
  order_id INT NOT NULL DEFAULT 0 COMMENT '訂單id，來源于order.id',
  goods_id INT NOT NULL DEFAULT 0 COMMENT '商品id，來源于goods.id',
  num INT NOT NULL DEFAULT 0 COMMENT '商品數量',
  total_price DECIMAL(12,2) NOT NULL DEFAULT 0 COMMENT '商品總金額'
) COMMENT '訂單表';
INSERT INTO order_detail VALUES (1,1,1,2,16.00),(2,1,1,1,16.00),(3,2,1,1,8.00);
select * from user;
select * from goods;
select * from orders;
select * from order_detail;

建好庫和表后，下一步我們就開始我們的查詢之旅，先從最基礎的單表查詢看起，依次介紹單表查詢、一對一查詢和一對多查詢，

單表查詢(3種方式)

先來介紹的就是單表查詢，

需求

需要按照訂單id查詢訂單資訊，

方式1

創建每個表對應的Model

db中表的欄位是采用下劃線分割的，model中我們是采用駱駝命名法來命名的，如OrderModel:

package com.zhonghu.chat07.demo1.model;
import lombok.*;
import java.util.List;
@Getter
@Setter
@Builder
@ToString
@NoArgsConstructor
@AllArgsConstructor
public class OrderModel {
    private Integer id;
    private Integer userId;
    private Long createTime;
    private Long upTime;
}

其他幾個Model也類似，

Mapper xml

<select id="getById" resultType="com.zhonghu.chat07.demo1.model.OrderModel">
    <![CDATA[
    SELECT a.id,a.user_id as userId,a.create_time createTime,a.up_time upTime FROM orders a WHERE a.id = #{value}
    ]]>
</select>

注意上面的resultType，標識結果的型別，

Mapper介面方法

OrderModel getById(int id);

mybatis全域組態檔

<?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>
    <!-- 引入外部jdbc配置 -->
    <properties resource="jdbc.properties"/>
    <!-- 環境配置，可以配置多個環境 -->
    <environments default="demo4">
        <environment id="demo4">
            <!-- 事務管理器工廠配置 -->
            <transactionManager type="JDBC"/>
            <!-- 資料源工廠配置，使用工廠來創建資料源 -->
            <dataSource type="POOLED">
                <property name="driver" value="https://www.cnblogs.com/javazhonghu/p/${jdbc.driver}"/>
                <property name="url" value="https://www.cnblogs.com/javazhonghu/p/${jdbc.url}"/>
                <property name="username" value="https://www.cnblogs.com/javazhonghu/p/${jdbc.username}"/>
                <property name="password" value="https://www.cnblogs.com/javazhonghu/p/${jdbc.password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="demo1/mapper/UserMapper.xml" />
        <mapper resource="demo1/mapper/GoodsMapper.xml" />
        <mapper resource="demo1/mapper/OrderMapper.xml" />
        <mapper resource="demo1/mapper/OrderDetailMapper.xml" />
    </mappers>
</configuration>

測驗用例

com.zhonghu.chat07.demo1.Demo1Test#getById
@Before
public void before() throws IOException {
    //指定mybatis全域組態檔
    String resource = "demo1/mybatis-config.xml";
    //讀取全域組態檔
    InputStream inputStream = Resources.getResourceAsStream(resource);
    //構建SqlSessionFactory物件
    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    this.sqlSessionFactory = sqlSessionFactory;
}
@Test
public void getById() {
    try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
        OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
        OrderModel orderModel = mapper.getById(1);
        log.info("{}", orderModel);
    }
}

運行輸出

35:59.211 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById - ==>  Preparing: SELECT a.id,a.user_id as userId,a.create_time createTime,a.up_time upTime FROM orders a WHERE a.id = ? 
35:59.239 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById - ==> Parameters: 1(Integer)
35:59.258 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById - <==      Total: 1
35:59.258 [main] INFO  c.j.chat05.demo1.Demo1Test - OrderModel(id=1, userId=2, createTime=1610803573, upTime=1610803573)

原理

sql中我們使用了別名，將orders中的欄位轉換成了和OrderModel中欄位一樣的名稱，最后mybatis內部會通過反射，將查詢結果按照名稱到OrderModel中查找同名的欄位，然后進行賦值，

方式2

若我們專案中表對應的Model中的欄位都是采用駱駝命名法，mybatis中可以進行一些配置，可以使表中的欄位和對應Model中駱駝命名法的欄位進行自動映射，

需要在mybatis全域組態檔中加入下面配置：

<settings>
    <!-- 是否開啟自動駝峰命名規則映射，及從xx_yy映射到xxYy -->
    <setting name="mapUnderscoreToCamelCase" value="https://www.cnblogs.com/javazhonghu/p/true"/>
</settings>

Mapper xml

<select id="getById1" resultType="com.zhonghu.chat07.demo1.model.OrderModel">
    <![CDATA[
    SELECT a.id,a.user_id,a.create_time,a.up_time FROM orders a WHERE a.id = #{value}
    ]]>
</select>

注意上面的sql，我們沒有寫別名了，由于我們開啟了自動駱駝命名映射，所以查詢結果會按照下面的關系進行自動映射：

sql對應的欄位	OrderModel中的欄位
id	id
user_id	userId
create_time	createTime
up_time	upTime

Mapper介面

OrderModel getById1(int id);

測驗用例

com.zhonghu.chat05.demo1.Demo1Test#getById1
@Test
public void getById1() {
    try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
        OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
        OrderModel orderModel = mapper.getById1(1);
        log.info("{}", orderModel);
    }
}

運行輸出

59:44.884 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - ==>  Preparing: SELECT a.id,a.user_id,a.create_time,a.up_time FROM orders a WHERE a.id = ? 
59:44.917 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - ==> Parameters: 1(Integer)
59:44.935 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - <==      Total: 1
59:44.935 [main] INFO  c.j.chat05.demo1.Demo1Test - OrderModel(id=1, userId=2, createTime=1610803573, upTime=1610803573)

輸出中可以看出，sql中的欄位是下劃線的方式，OrderModel中的欄位是駱駝命名法，結果也自動裝配成功，這個就是開啟mapUnderscoreToCamelCase產生的效果，

方式3

mapper xml中有個更強大的元素resultMap，通過這個元素可以定義查詢結果的映射關系，

Mapper xml

<resultMap id="orderModelMap2" type="com.zhonghu.chat07.demo1.model.OrderModel">
    <id column="id" property="id" />
    <result column="user_id" property="userId" />
    <result column="create_time" property="createTime" />
    <result column="up_time" property="upTime" />
</resultMap>
<select id="getById2" resultMap="orderModelMap2">
    <![CDATA[
    SELECT a.id,a.user_id,a.create_time,a.up_time FROM orders a WHERE a.id = #{value}
    ]]>
</select>

上面resultMap有2個元素需要指定：

id：resultMap標識
type：將結果封裝成什么型別，此處我們需要將結果分裝為OrderModel

注意上面的select元素，有個resultMap，標識查詢結果使用哪個resultMap進行映射，此處我們使用的是orderModelMap2，所以查詢結果會按照orderModelMap2關聯的resultMap進行映射，

Mapper介面

OrderModel getById2(int id);

測驗用例

com.zhonghu.chat07.demo1.Demo1Test#getById2
@Test
public void getById2() {
    try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
        OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
        OrderModel orderModel = mapper.getById2(1);
        log.info("{}", orderModel);
    }
}

運行輸出

14:12.518 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - ==>  Preparing: SELECT a.id,a.user_id,a.create_time,a.up_time FROM orders a WHERE a.id = ? 
14:12.546 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - ==> Parameters: 1(Integer)
14:12.564 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - <==      Total: 1
14:12.564 [main] INFO  c.j.chat05.demo1.Demo1Test - OrderModel(id=1, userId=2, createTime=1610803573, upTime=1610803573)

一對一關聯查詢(4種方式)

講完了單表查詢，下面開始連表查詢，首先就是一對一的查詢

需求

通過訂單id查詢訂單的時候，將訂單關聯的用戶資訊也回傳，

我們修改一下OrderModel代碼，內部添加一個UserModel，如下：

package com.zhonghu.chat07.demo2.model;
import lombok.*;
import java.util.List;
@Getter
@Setter
@Builder
@ToString
@NoArgsConstructor
@AllArgsConstructor
public class OrderModel {
    private Integer id;
    private Integer userId;
    private Long createTime;
    private Long upTime;
    //下單用戶資訊
    private UserModel userModel;
}

UserModel內容：

package com.zhonghu.chat07.demo2.model;
import lombok.*;
@Getter
@Setter
@Builder
@ToString
@NoArgsConstructor
@AllArgsConstructor
public class UserModel {
    private Integer id;
    private String name;
}

方式1

Mapper xml

<resultMap id="orderModelMap1" type="com.zhonghu.chat07.demo2.model.OrderModel">
    <id column="id" property="id" />
    <result column="user_id" property="userId"/>
    <result column="create_time" property="createTime"/>
    <result column="up_time" property="upTime"/>
    <result column="user_id" property="userModel.id"/>
    <result column="name" property="userModel.name"/>
</resultMap>
<select id="getById1" resultMap="orderModelMap1">
    <![CDATA[
    SELECT
        a.id,
        a.user_id,
        a.create_time,
        a.up_time,
        b.name
    FROM
        orders a,
        user b
    WHERE
        a.user_id = b.id
    AND a.id = #{value}
    ]]>
</select>

注意重點在于上面的這兩行：

<result column="user_id" property="userModel.id"/>
<result column="name" property="userModel.name"/>

這個地方使用到了級聯賦值，多級之間用.進行參考，此處我們只有一級，可以有很多級，

Mapper 介面

OrderModel getById1(int id);

測驗用例

com.zhonghu.chat07.demo2.Demo2Test#getById1
@Before
public void before() throws IOException {
    //指定mybatis全域組態檔
    String resource = "demo2/mybatis-config.xml";
    //讀取全域組態檔
    InputStream inputStream = Resources.getResourceAsStream(resource);
    //構建SqlSessionFactory物件
    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    this.sqlSessionFactory = sqlSessionFactory;
}
@Test
public void getById1() {
    try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
        OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
        OrderModel orderModel = mapper.getById1(1);
        log.info("{}", orderModel);
    }
}

運行輸出

24:20.811 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - ==>  Preparing: SELECT a.id, a.user_id, a.create_time, a.up_time, b.name FROM orders a, user b WHERE a.user_id = b.id AND a.id = ? 
24:20.843 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - ==> Parameters: 1(Integer)
24:20.861 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - <==      Total: 1
24:20.861 [main] INFO  c.j.chat05.demo2.Demo2Test - OrderModel(id=1, userId=2, createTime=1610803573, upTime=1610803573, userModel=UserModel(id=2, name=Java冢狐))

方式2

這次我們需要使用mapper xml中另外一個元素association，這個元素可以配置關聯物件的映射關系，看示例，

Mapper xml

<resultMap id="orderModelMap2" type="com.zhonghu.chat07.demo2.model.OrderModel">
    <id column="id" property="id" />
    <result column="user_id" property="userId"/>
    <result column="create_time" property="createTime"/>
    <result column="up_time" property="upTime"/>
    <association property="userModel">
        <id column="user_id" property="id"/>
        <result column="name" property="name" />
    </association>
</resultMap>
<select id="getById2" resultMap="orderModelMap2">
    <![CDATA[
    SELECT
        a.id,
        a.user_id,
        a.create_time,
        a.up_time,
        b.name
    FROM
        orders a,
        user b
    WHERE
        a.user_id = b.id
    AND a.id = #{value}
    ]]>
</select>

注意上面下面這部分代碼：

<association property="userModel">
    <id column="user_id" property="id"/>
    <result column="name" property="name" />
</association>

注意上面的property屬性，這個就是配置sql查詢結果和OrderModel.userModel物件的映射關系，將user_id和userModel中的id進行映射,name和userModel中的name進行映射，

Mapper介面

OrderModel getById2(int id);

測驗用例

@Test
public void getById2() {
    try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
        OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
        OrderModel orderModel = mapper.getById2(1);
        log.info("{}", orderModel);
    }
}

運行結果

51:44.896 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - ==>  Preparing: SELECT a.id, a.user_id, a.create_time, a.up_time, b.name FROM orders a, user b WHERE a.user_id = b.id AND a.id = ? 
51:44.925 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - ==> Parameters: 1(Integer)
51:44.941 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - <==      Total: 1
51:44.942 [main] INFO  c.j.chat05.demo2.Demo2Test - OrderModel(id=1, userId=2, createTime=1610803573, upTime=1610803573, userModel=UserModel(id=2, name=Java冢狐))

從結果的最后一行可以看出，所有欄位的值映射都是ok的，

方式3

先按照訂單id查詢訂單資料，然后在通過訂單中user_id去用戶表查詢用戶資料，通過兩次查詢，組合成目標結果，mybatis已經內置了這種操作，如下，

UserMapper.xml

我們先定義一個通過用戶id查詢用戶資訊的select元素，如下

<select id="getById" resultType="com.zhonghu.chat07.demo2.model.UserModel">
    <![CDATA[
    SELECT id,name FROM user where id = #{value}
    ]]>
</select>

OrderModel.xml

<resultMap id="orderModelMap3" type="com.zhonghu.chat07.demo2.model.OrderModel">
    <id column="id" property="id" />
    <result column="user_id" property="userId"/>
    <result column="create_time" property="createTime"/>
    <result column="up_time" property="upTime"/>
    <association property="userModel" select="com.zhonghu.chat07.demo2.mapper.UserMapper.getById" column="user_id" />
</resultMap>
<select id="getById3" resultMap="orderModelMap3">
    <![CDATA[
    SELECT
        a.id,
        a.user_id,
        a.create_time,
        a.up_time
    FROM
        orders a
    WHERE
        a.id = #{value}
    ]]>
</select>

OrderModel.userModel屬性的值來在于另外一個查詢，這個查詢是通過association元素的select屬性指定的，此處使用的是

com.zhonghu.chat07.demo2.mapper.UserMapper.getById

這個查詢是有條件的，條件通過association的column進行傳遞的，此處傳遞的是getById3查詢結果中的user_id欄位，

Mapper介面

OrderModel getById3(int id);

測驗用例

com.zhonghu.chat07.demo2.Demo2Test#getById3
@Test
public void getById3() {
    try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
        OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
        OrderModel orderModel = mapper.getById3(1);
        log.info("{}", orderModel);
    }
}

運行輸出

07:12.569 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById3 - ==>  Preparing: SELECT a.id, a.user_id, a.create_time, a.up_time FROM orders a WHERE a.id = ? 
07:12.600 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById3 - ==> Parameters: 1(Integer)
07:12.619 [main] DEBUG c.j.c.d.mapper.UserMapper.getById - ====>  Preparing: SELECT id,name FROM user where id = ? 
07:12.620 [main] DEBUG c.j.c.d.mapper.UserMapper.getById - ====> Parameters: 2(Integer)
07:12.625 [main] DEBUG c.j.c.d.mapper.UserMapper.getById - <====      Total: 1
07:12.625 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById3 - <==      Total: 1
07:12.625 [main] INFO  c.j.chat05.demo2.Demo2Test - OrderModel(id=1, userId=2, createTime=1610803573, upTime=1610803573, userModel=UserModel(id=2, name=Java冢狐))

從輸出中可以看出有2次查詢，先按照訂單id查詢訂單，然后通過訂單記錄中用戶id去用戶表查詢用戶資訊，最終執行了2次查詢，

方式4

方式3中給第二個查詢傳遞了一個引數，如果需要給第二個查詢傳遞多個引數怎么辦呢？可以這么寫

<association property="屬性" select="查詢對應的select的id" column="{key1=父查詢欄位1,key2=父查詢欄位2,key3=父查詢欄位3}" />

這種相當于給子查詢傳遞了一個map，子查詢中需要用過map的key獲取對應的條件，看案例：

OrderMapper.xml

<resultMap id="orderModelMap4" type="com.zhonghu.chat07.demo2.model.OrderModel">
    <id column="id" property="id" />
    <result column="user_id" property="userId"/>
    <result column="create_time" property="createTime"/>
    <result column="up_time" property="upTime"/>
    <association property="userModel" select="com.zhonghu.chat07.demo2.mapper.UserMapper.getById1" column="{uid1=user_id,uid2=create_time}" />
</resultMap>
<select id="getById4" resultMap="orderModelMap4">
    <![CDATA[
    SELECT
        a.id,
        a.user_id,
        a.create_time,
        a.up_time
    FROM
        orders a
    WHERE
        a.id = #{value}
    ]]>
</select>

UserMapper.xml

<select id="getById1" resultType="com.zhonghu.chat07.demo2.model.UserModel">
    <![CDATA[
    SELECT id,name FROM user where id = #{uid1} and id = #{uid2}
    ]]>
</select>

Mapper介面

OrderModel getById4(int id);

測驗用例

com.zhonghu.chat07.demo2.Demo2Test#getById4
@Test
public void getById4() {
    try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
        OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
        OrderModel orderModel = mapper.getById4(1);
        log.info("{}", orderModel);
    }
}

運行輸出

19:59.881 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById4 - ==>  Preparing: SELECT a.id, a.user_id, a.create_time, a.up_time FROM orders a WHERE a.id = ? 
19:59.914 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById4 - ==> Parameters: 1(Integer)
19:59.934 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - ====>  Preparing: SELECT id,name FROM user where id = ? and id = ? 
19:59.934 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - ====> Parameters: 2(Integer), 1610803573(Long)
19:59.939 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - <====      Total: 0
19:59.939 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById4 - <==      Total: 1
19:59.939 [main] INFO  c.j.chat05.demo2.Demo2Test - OrderModel(id=1, userId=2, createTime=1610803573, upTime=1610803573, userModel=null)

輸出中看一下第二個查詢的條件，傳過來的是第一個查詢的user_id和create_time，

一對多查詢(2種方式)

最后就是進行一隊多查詢的決議

需求

根據訂單id查詢出訂單資訊，并且查詢出訂單明細串列，

先修改一下OrderModel代碼，如下：

package com.zhonghu.chat07.demo3.model;
import lombok.*;
import java.util.List;
@Getter
@Setter
@Builder
@ToString
@NoArgsConstructor
@AllArgsConstructor
public class OrderModel {
    private Integer id;
    private Integer userId;
    private Long createTime;
    private Long upTime;
    //訂單詳情串列
    private List<OrderDetailModel> orderDetailModelList;
}

OrderModel中添加了一個集合orderDetailModelList用來存放訂單詳情串列，

方式1

OrderMapper.xml

<resultMap id="orderModelMap1" type="com.zhonghu.chat07.demo3.model.OrderModel">
    <id column="id" property="id"/>
    <result column="user_id" property="userId"/>
    <result column="create_time" property="createTime"/>
    <result column="up_time" property="upTime"/>
    <collection property="orderDetailModelList" ofType="com.zhonghu.chat07.demo3.model.OrderDetailModel">
        <id column="orderDetailId" property="id"/>
        <result column="order_id" property="orderId"/>
        <result column="goods_id" property="goodsId"/>
        <result column="num" property="num"/>
        <result column="total_price" property="totalPrice"/>
    </collection>
</resultMap>
<select id="getById1" resultMap="orderModelMap1">
    <![CDATA[
    SELECT
        a.id ,
        a.user_id,
        a.create_time,
        a.up_time,
        b.id orderDetailId,
        b.order_id,
        b.goods_id,
        b.num,
        b.total_price
    FROM
        orders a,
        order_detail b
    WHERE
        a.id = b.order_id
        AND a.id = #{value}
    ]]>
</select>

注意上面的getById1中的sql，這個sql中使用到了t_order和t_order_detail連接查詢，這個查詢會回傳多條結果，但是最后結果按照orderModelMap1進行映射，最后只會回傳一個OrderModel物件，關鍵在于collection元素，這個元素用來定義集合中元素的映射關系，有2個屬性需要注意：

property：對應的屬性名稱
ofType：集合中元素的型別，此處是OrderDetailModel

原理是這樣的，注意orderModelMap1中有個

<id column="id" property="id"/>

查詢出來的結果會按照這個配置中指定的column進行分組，即按照訂單id進行分組，每個訂單對應多個訂單明細，訂單明細會按照collection的配置映射為ofType元素指定的物件，

實際resultMap元素中的id元素可以使用result元素代替，只是用id可以提升性能，mybatis可以通過id元素配置的列的值判斷唯一一條記錄，如果我們使用result元素，那么判斷是否是同一條記錄的時候，需要通過所有列去判斷了，所以通過id可以提升性能，使用id元素在一對多中可以提升性能，在單表查詢中使用id元素還是result元素，性能都是一樣的，

Mapper介面

OrderModel getById1(Integer id);

測驗用例

com.zhonghu.chat07.demo3.Demo3Test#getById1
@Before
public void before() throws IOException {
    //指定mybatis全域組態檔
    String resource = "demo3/mybatis-config.xml";
    //讀取全域組態檔
    InputStream inputStream = Resources.getResourceAsStream(resource);
    //構建SqlSessionFactory物件
    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    this.sqlSessionFactory = sqlSessionFactory;
}
@Test
public void getById1() {
    try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
        OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
        Integer id = 1;
        OrderModel orderModel = mapper.getById1(id);
        log.info("{}", orderModel);
    }
}

運行輸出

03:52.092 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - ==>  Preparing: SELECT a.id , a.user_id, a.create_time, a.up_time, b.id orderDetailId, b.order_id, b.goods_id, b.num, b.total_price FROM orders a, order_detail b WHERE a.id = b.order_id AND a.id = ? 
03:52.124 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - ==> Parameters: 1(Integer)
03:52.148 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - <==      Total: 2
03:52.148 [main] INFO  c.j.chat05.demo3.Demo3Test - OrderModel(id=1, userId=2, createTime=1610803573, upTime=1610803573, orderDetailModelList=[OrderDetailModel(id=1, orderId=1, goodsId=1, num=2, totalPrice=16.00), OrderDetailModel(id=2, orderId=1, goodsId=1, num=1, totalPrice=16.00)])

注意最后一條輸出，和期望的結果一致，

方式2

通過2次查詢，然后對結果進行分裝，先通過訂單id查詢訂單資訊，然后通過訂單id查詢訂單明細串列，然后封裝結果，mybatis中默認支持這么玩，還是通過collection元素來實作的，

OrderDetailMapper.xml

<select id="getListByOrderId1" resultType="com.zhonghu.chat07.demo3.model.OrderDetailModel" parameterType="int">
    <![CDATA[
    SELECT
        a.id,
        a.order_id AS orderId,
        a.goods_id AS goodsId,
        a.num,
        a.total_price AS totalPrice
    FROM
        order_detail a
    WHERE
        a.order_id = #{value}
    ]]>
</select>

OrderMapper.xml

<resultMap id="orderModelMap2" type="com.zhonghu.chat07.demo3.model.OrderModel">
    <id column="id" property="id"/>
    <result column="user_id" property="userId"/>
    <result column="create_time" property="createTime"/>
    <result column="up_time" property="upTime"/>
    <collection property="orderDetailModelList" select="com.zhonghu.chat07.demo3.mapper.OrderDetailMapper.getListByOrderId1" column="id"/>
</resultMap>
<select id="getById2" resultMap="orderModelMap2">
    <![CDATA[
    SELECT
        a.id ,
        a.user_id,
        a.create_time,
        a.up_time
    FROM
        orders a
    WHERE
        a.id = #{value}
    ]]>
</select>

重點在于下面這句配置：

<collection property="orderDetailModelList" select="com.zhonghu.chat07.demo3.mapper.OrderDetailMapper.getListByOrderId1" column="id"/>

表示orderDetailModelList屬性的值通過select屬性指定的查詢獲取，即：

com.zhonghu.chat07.demo3.mapper.OrderDetailMapper.getListByOrderId1

查詢引數是通過column屬性指定的，此處使用getById2 sql中的id作為條件，即訂單id，

Mapper介面

OrderModel getById2(int id);

測驗用例

com.zhonghu.chat07.demo3.Demo3Test#getById2
@Test
public void getById2() {
    try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
        OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
        OrderModel orderModel = mapper.getById2(1);
        log.info("{}", orderModel);
    }
}

運行輸出

10:07.087 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - ==>  Preparing: SELECT a.id , a.user_id, a.create_time, a.up_time FROM ordera a WHERE a.id = ? 
10:07.117 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - ==> Parameters: 1(Integer)
10:07.135 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - ====>  Preparing: SELECT a.id, a.order_id AS orderId, a.goods_id AS goodsId, a.num, a.total_price AS totalPrice FROM order_detail a WHERE a.order_id = ? 
10:07.136 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - ====> Parameters: 1(Integer)
10:07.141 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - <====      Total: 2
10:07.142 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - <==      Total: 1
10:07.142 [main] INFO  c.j.chat05.demo3.Demo3Test - OrderModel(id=1, userId=2, createTime=1610803573, upTime=1610803573, orderDetailModelList=[OrderDetailModel(id=1, orderId=1, goodsId=1, num=2, totalPrice=16.00), OrderDetailModel(id=2, orderId=1, goodsId=1, num=1, totalPrice=16.00)])

輸出中有2次查詢，先通過訂單id查詢訂單資訊，然后通過訂單id查詢訂單明細，mybatis內部對結果進行了組裝，

總結

mybatis全域組態檔中通過mapUnderscoreToCamelCase可以開啟sql中的欄位和javabean中的駱駝命名法的欄位進行自動映射
掌握resultMap元素常見的用法
一對一關聯查詢使用resultMap->association元素（2種方式）
一對多查詢使用resultMap->collection元素（2種方式）
resultMap中使用id元素主要在復雜的關聯查詢中可以提升效率，可以通過這個來判斷記錄的唯一性，如果沒有這個，需要通過所有的result相關的列才能判斷記錄的唯一性

建議

mybatis為我們提供了強大的關聯查詢，不過個人建議盡量少用，最好是采用單表的方式查詢，在程式中通過多次查詢，然后自己對結果進行組裝，

Model中最好只定義一些和單表欄位關聯的屬性，不要摻雜著其他物件的參考，

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