Mybatis（第二篇：联表查询）

目录

  • Mybatis（第二篇：联表查询）
    • 一、前期 项目的搭建
      • 1.数据库
      • 2.IDEA项目架构搭建
        • 2.1 pom.xml
        • 2.2 domain包
          • 2.2.1 Department.java
          • 2.2.2 Employee.java
          • 2.2.3 Role.java
        • 2.3 util包
          • MybatisUtil.java
        • 2.4 mybatis-config.xml
        • 2.5 db.properties
        • 2.6 log4j.properties
        • 2.7 mapper包
          • 2.7.1 departmentMapper.java
          • 2.7.2 EmployeeMapper.java
          • 2.7.3 RoleMapper.java
        • 2.8 xml
          • 2.8.1 DepartmentMapper.xml
            • 部分代码说明——resultMap的两种写法
            • 全部代码
          • 2.8.2 EmployeeMapper.xml
          • 2.8.3 RoleMapper.xml
        • 2.9 测试类
    • 3. 过程知识点细讲
      • 3.1 一对多封装 （一个员工对应一个部门）
        • 方案一:关联查询（association、property、javaType、[columnPrefix]）
          • columnPrefix
        • 方案二：发送额外sql（1）（推荐使用这种）
        • 方案二：发送额外sql（2）
        • 两种方案的对比
      • 3.2 多对多封装（员工和角色）
        • 方案一：发送额外SQL （1）（推荐使用这种方式）
          • 是否发送额外SQL 配置懒加载
        • 方案一：发送额外SQL（2）
        • 方案二：关联查询
    • 总结

一、前期 项目的搭建

1.数据库

user.sql

/*Navicat MySQL Data TransferSource Server         : localhost_3306Source Server Type    : MySQLSource Server Version : 50736Source Host           : localhost:3306Source Schema         : demoTarget Server Type    : MySQLTarget Server Version : 50736File Encoding         : 65001Date: 17/06/2022 09:52:53
*/SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;-- ----------------------------
-- Table structure for user
-- ----------------------------
DROP TABLE IF EXISTS `user`;
CREATE TABLE `user`  (`id` bigint(20) NOT NULL AUTO_INCREMENT,`name` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,`age` int(11) NULL DEFAULT NULL,`sex` char(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,`address` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,`birthday` date NULL DEFAULT NULL,PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 4 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;-- ----------------------------
-- Records of user
-- ----------------------------
INSERT INTO `user` VALUES (1, '小明', 20, '男', '武汉', '2022-06-10');
INSERT INTO `user` VALUES (2, '小红', 18, '女', '深圳', '2022-06-08');
INSERT INTO `user` VALUES (3, '小丽', 17, '女', '北京', '2019-12-02');SET FOREIGN_KEY_CHECKS = 1;

department.sql

/*Navicat MySQL Data TransferSource Server         : localhost_3306Source Server Type    : MySQLSource Server Version : 50736Source Host           : localhost:3306Source Schema         : crmTarget Server Type    : MySQLTarget Server Version : 50736File Encoding         : 65001Date: 20/06/2022 17:33:24
*/SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;-- ----------------------------
-- Table structure for department
-- ----------------------------
DROP TABLE IF EXISTS `department`;
CREATE TABLE `department`  (`id` bigint(20) NOT NULL AUTO_INCREMENT,`name` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,`sn` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 7 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;-- ----------------------------
-- Records of department
-- ----------------------------
INSERT INTO `department` VALUES (1, '总经办2', 'General Deparment2');
INSERT INTO `department` VALUES (2, '人力资源部', 'Human Resources Department');
INSERT INTO `department` VALUES (3, '采购部', 'Order Department');
INSERT INTO `department` VALUES (4, '仓储部', 'Warehousing Department');
INSERT INTO `department` VALUES (6, '技术部', 'Technolog Department ');SET FOREIGN_KEY_CHECKS = 1;

employee.sql

/*Navicat MySQL Data TransferSource Server         : localhost_3306Source Server Type    : MySQLSource Server Version : 50736Source Host           : localhost:3306Source Schema         : crmTarget Server Type    : MySQLTarget Server Version : 50736File Encoding         : 65001Date: 20/06/2022 17:33:32
*/SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;-- ----------------------------
-- Table structure for employee
-- ----------------------------
DROP TABLE IF EXISTS `employee`;
CREATE TABLE `employee`  (`id` bigint(20) NOT NULL AUTO_INCREMENT,`username` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,`password` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,`email` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,`age` int(11) NULL DEFAULT NULL,`admin` bit(1) NULL DEFAULT NULL,`dept_id` bigint(20) NULL DEFAULT NULL,PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 20 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;-- ----------------------------
-- Records of employee
-- ----------------------------
INSERT INTO `employee` VALUES (1, 'admin', '$2a$10$kYaoE4sx1uweWHoJt1CExO7ZpaM4Akt4rPnOcXK9/KRWxRuuEIfWC', 'admin@abc.com', 20, b'1', 2);
INSERT INTO `employee` VALUES (2, 'zhao', '$2a$10$kYaoE4sx1uweWHoJt1CExO7ZpaM4Akt4rPnOcXK9/KRWxRuuEIfWC', 'zhaoz@wolfcode.cn', 35, b'0', 1);
INSERT INTO `employee` VALUES (3, '赵一明', 'c4ca4238a0b923820dcc509a6f75849b', 'zhaoym@wolfcode.cn', 25, b'0', 1);
INSERT INTO `employee` VALUES (4, '钱总', 'c4ca4238a0b923820dcc509a6f75849b', 'qianz@wolfcode.cn', 35, b'0', 2);
INSERT INTO `employee` VALUES (5, '钱二明', 'c4ca4238a0b923820dcc509a6f75849b', 'qianem@wolfcode.cn', 25, b'0', 2);
INSERT INTO `employee` VALUES (6, '孙总', 'c4ca4238a0b923820dcc509a6f75849b', 'sunz@wolfcode.cn', 35, b'0', 3);
INSERT INTO `employee` VALUES (7, '孙三明', 'c4ca4238a0b923820dcc509a6f75849b', 'sunsm@wolfcode.cn', 25, b'0', 3);
INSERT INTO `employee` VALUES (9, '李四明', 'c4ca4238a0b923820dcc509a6f75849b', 'lism@wolfcode.cn', 25, b'0', 4);
INSERT INTO `employee` VALUES (10, '周总', 'c4ca4238a0b923820dcc509a6f75849b', 'zhouz@wolfcode.cn', 35, b'0', 5);
INSERT INTO `employee` VALUES (11, '周五明', 'c4ca4238a0b923820dcc509a6f75849b', 'zhouwm@wolfcode.cn', 25, b'0', 5);
INSERT INTO `employee` VALUES (12, '吴总', 'c4ca4238a0b923820dcc509a6f75849b', 'wuz@wolfcode.cn', 35, b'0', 6);
INSERT INTO `employee` VALUES (13, '吴六明', 'c4ca4238a0b923820dcc509a6f75849b', 'wulm@wolfcode.cn', 25, b'0', 6);
INSERT INTO `employee` VALUES (14, '郑总', 'c4ca4238a0b923820dcc509a6f75849b', 'zhengz@wolfcode.cn', 35, b'0', 7);
INSERT INTO `employee` VALUES (15, '郑七明', 'c4ca4238a0b923820dcc509a6f75849b', 'zhengqm@wolfcode.cn', 25, b'0', 7);
INSERT INTO `employee` VALUES (16, '孙四明', 'c4ca4238a0b923820dcc509a6f75849b', 'sunsim@wolfcode.cn', 25, b'0', 3);
INSERT INTO `employee` VALUES (17, '孙五明2', 'c4ca4238a0b923820dcc509a6f75849b', 'sunwm@wolfcode.cn', 25, b'0', 1);
INSERT INTO `employee` VALUES (18, '李五明', 'c4ca4238a0b923820dcc509a6f75849b', 'liwm@wolfcode.cn', 25, b'0', 4);
INSERT INTO `employee` VALUES (19, '李六明', 'c4ca4238a0b923820dcc509a6f75849b', 'lilm@wolfcode.cn', 25, b'0', 4);SET FOREIGN_KEY_CHECKS = 1;

role.sql

/*Navicat MySQL Data TransferSource Server         : localhost_3306Source Server Type    : MySQLSource Server Version : 50736Source Host           : localhost:3306Source Schema         : crmTarget Server Type    : MySQLTarget Server Version : 50736File Encoding         : 65001Date: 20/06/2022 17:33:49
*/SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;-- ----------------------------
-- Table structure for role
-- ----------------------------
DROP TABLE IF EXISTS `role`;
CREATE TABLE `role`  (`id` bigint(20) NOT NULL AUTO_INCREMENT,`name` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,`sn` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 15 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;-- ----------------------------
-- Records of role
-- ----------------------------
INSERT INTO `role` VALUES (1, '人事管理', 'HR_MGR');
INSERT INTO `role` VALUES (2, '采购管理', 'ORDER_MGR');
INSERT INTO `role` VALUES (3, '仓储管理', 'WAREHOUSING_MGR');
INSERT INTO `role` VALUES (4, '行政部管理', 'Admin_MGR');
INSERT INTO `role` VALUES (11, '市场经理', 'Market_Manager');
INSERT INTO `role` VALUES (12, '市场专员', 'Market');
INSERT INTO `role` VALUES (13, '2222222', '22222222');
INSERT INTO `role` VALUES (14, '1231', '3132');SET FOREIGN_KEY_CHECKS = 1;

employee_role.sql

/*Navicat MySQL Data TransferSource Server         : localhost_3306Source Server Type    : MySQLSource Server Version : 50736Source Host           : localhost:3306Source Schema         : crmTarget Server Type    : MySQLTarget Server Version : 50736File Encoding         : 65001Date: 20/06/2022 17:33:41
*/SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;-- ----------------------------
-- Table structure for employee_role
-- ----------------------------
DROP TABLE IF EXISTS `employee_role`;
CREATE TABLE `employee_role`  (`employee_id` bigint(20) NULL DEFAULT NULL,`role_id` bigint(20) NULL DEFAULT NULL
) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;-- ----------------------------
-- Records of employee_role
-- ----------------------------
INSERT INTO `employee_role` VALUES (2, 1);
INSERT INTO `employee_role` VALUES (3, 12);
INSERT INTO `employee_role` VALUES (5, 11);
INSERT INTO `employee_role` VALUES (22, 11);
INSERT INTO `employee_role` VALUES (22, 12);
INSERT INTO `employee_role` VALUES (23, 3);
INSERT INTO `employee_role` VALUES (23, 4);
INSERT INTO `employee_role` VALUES (23, 11);
INSERT INTO `employee_role` VALUES (23, 12);
INSERT INTO `employee_role` VALUES (17, 1);
INSERT INTO `employee_role` VALUES (17, 2);
INSERT INTO `employee_role` VALUES (17, 3);
INSERT INTO `employee_role` VALUES (17, 4);
INSERT INTO `employee_role` VALUES (17, 11);
INSERT INTO `employee_role` VALUES (17, 12);
INSERT INTO `employee_role` VALUES (17, 13);SET FOREIGN_KEY_CHECKS = 1;

2.IDEA项目架构搭建

2.1 pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion><groupId>com.bjpowernode</groupId><artifactId>02_realtionproject</artifactId><version>1.0.0</version><properties><project.build.sourceEncoding>UTF-8</project.build.sourceEncoding><maven.compiler.source>1.8</maven.compiler.source><maven.compiler.target>1.8</maven.compiler.target></properties><dependencies><dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><version>8.0.28</version></dependency><dependency><groupId>org.mybatis</groupId><artifactId>mybatis</artifactId><version>3.5.9</version></dependency><dependency><groupId>junit</groupId><artifactId>junit</artifactId><version>4.12</version><scope>test</scope></dependency><dependency><groupId>org.projectlombok</groupId><artifactId>lombok</artifactId><version>1.18.20</version></dependency><!--log--><dependency><groupId>commons-logging</groupId><artifactId>commons-logging</artifactId><version>1.2</version></dependency><dependency><groupId>org.apache.logging.log4j</groupId><artifactId>log4j-core</artifactId><version>2.13.3</version></dependency><dependency><groupId>log4j</groupId><artifactId>log4j</artifactId><version>1.2.17</version></dependency><!-- 分页插件jar --><dependency><groupId>com.github.pagehelper</groupId><artifactId>pagehelper</artifactId><version>5.2.0</version></dependency></dependencies></project>

2.2 domain包

2.2.1 Department.java
package com.bjpowernode.domain;
import lombok.Data;@Data
public class Department {private Long id;private String name;private String sn;}
2.2.2 Employee.java
package com.bjpowernode.domain;import lombok.Data;
import lombok.Getter;
import lombok.Setter;
import lombok.ToString;import java.util.ArrayList;
import java.util.List;@Getter
@Setter
public class Employee {private Long id;private String username;private String name;private String password;private String email;private Integer age;private boolean admin;private Department dept; // 所属部门private List<Role> roles =new ArrayList(); //拥有的角色@Overridepublic String toString() {return "Employee{" +"id=" + id +", username='" + username + '\'' +", name='" + name + '\'' +", password='" + password + '\'' +", email='" + email + '\'' +", age=" + age +", admin=" + admin +", dept=" + dept +'}';}
}
2.2.3 Role.java
package com.bjpowernode.domain;import lombok.Data;@Data
public class Role {private Long id;private String name;private String sn;
}

2.3 util包

MybatisUtil.java
 package com.bjpowernode.util;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 java.io.IOException;
import java.io.InputStream;public class MyBatisUtil {private MyBatisUtil(){}private static SqlSessionFactory factory ;static {try {InputStream resourceAsStream = Resources.getResourceAsStream("mybatis-config.xml");factory=  new SqlSessionFactoryBuilder().build(resourceAsStream);} catch (IOException e) {e.printStackTrace();}}public static SqlSession openSession() {return factory.openSession(true);}
}

2.4 mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configurationPUBLIC "-//mybatis.org//DTD Config 3.0//EN""http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration><!--关联数据库的配置信息--><properties resource="db.properties"/><!--设置日志--><settings><setting name="logImpl" value="LOG4J"/><!-- 设置为 false 表示每个延迟加载属性会按需加载 --><setting name="aggressiveLazyLoading" value="false" /><!--是否开启懒加载 总开关 延迟加载的全局开关--><setting name="lazyLoadingEnabled" value="true"/><!--指定对象的哪些方法触发一次延迟加载--><setting name="lazyLoadTriggerMethods" value="clone"/></settings><!--给所有的 javaBean 起别名--><typeAliases><package name="com.bjpowernode.domain"/></typeAliases><!--分页插件的 配置--><plugins><plugin interceptor="com.github.pagehelper.PageInterceptor"><!--通过这个配置，来动态添加分页的功能--><property name="helperDialect" value="mysql"/><!--通过pageSizeZero属性可以灵活处理分页 当pageSize 设置为0就查询全部数据--><property name="pageSizeZero" value="true"/><!--通过reasonable 属性 可以合理化分页--><property name="reasonable" value="true"/></plugin></plugins><!--配置环境--><environments default="dev"><environment id="dev"><transactionManager type="JDBC"/><dataSource type="POOLED"><property name="driver" value="${driverClassName}"/><property name="url" value="${url}"/><property name="username" value="${username}"/><property name="password" value="${password}"/></dataSource></environment></environments><!--挂载sql 的映射文件--><mappers><mapper resource="com/bjpowernode/mapper/DepartmentMapper.xml"/><mapper resource="com/bjpowernode/mapper/EmployeeMapper.xml"/><mapper resource="com/bjpowernode/mapper/RoleMapper.xml"/></mappers></configuration>

2.5 db.properties

driverClassName=com.mysql.jdbc.Driver
url=jdbc:mysql:///rbac?useSSL=false
username=root
password=kong1234

2.6 log4j.properties

log4j.rootLogger=ERROR, stdoutlog4j.logger.com.bjpowernode.mapper=TRACElog4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p [%t] - %m%n

2.7 mapper包

2.7.1 departmentMapper.java
package com.bjpowernode.mapper;import com.bjpowernode.domain.Department;import java.util.List;public interface DepartmentMapper {List<Department>  selectAll();// 根据id 查询部门Department selectDeptById();
}
2.7.2 EmployeeMapper.java
package com.bjpowernode.mapper;import com.bjpowernode.domain.Employee;import java.util.List;public interface EmployeeMapper {List<Employee>  selectAll();// 关联查询List<Employee> selectEmpWithDepart();// 额外sqlList<Employee> selectEmpWithDepart2();// 额外sqlList<Employee> selectEmpWithRoles();}
2.7.3 RoleMapper.java
package com.bjpowernode.mapper;import com.bjpowernode.domain.Department;
import com.bjpowernode.domain.Role;import java.util.List;public interface RoleMapper {List<Role>  selectAll();List<Role> selectRolesByEmpId(Long empId);}

2.8 xml

2.8.1 DepartmentMapper.xml
部分代码说明——resultMap的两种写法

写法一：

  <resultMap id="baseMap" type="employee"><id column="id" property="id"/><result column="username" property="username"/><result column="password" property="password"/><result column="email" property="email"/><result column="age" property="age"/><result column="admin" property="admin"/><!--association用来封装对象属性的property 对象属性的名称（module层外键的那个属性 Employee的外键属性）javaType 对象属性的全限定名 (外键属性的全限定名)--><!--     <association property="department" javaType="com.bjpowernode.domain.Department">--><!--           <id column="d_id" property="id"/>--><!--          <result column="d_name" property="name"/>--><!--      </association>--><association property="department" javaType="com.bjpowernode.domain.Department" columnPrefix="d_"><id column="id" property="id"/><result column="name" property="name"/></association></resultMap>

写法二：

 <resultMap id="baseMap" type="employee"><id column="id" property="id"/><result column="username" property="username"/><result column="password" property="password"/><result column="email" property="email"/><result column="age" property="age"/><result column="admin" property="admin"/><result column="d_id" property="department.id"/><result column="d_name" property="department.name"/></resultMap>
全部代码
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.bjpowernode.mapper.DepartmentMapper"><select id="selectAll" resultType="department">select * from department
</select><select id="selectDeptById" resultType="department">select * from department where id = #{id}
</select></mapper>
2.8.2 EmployeeMapper.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.bjpowernode.mapper.EmployeeMapper"><select id="selectAll" resultType="employee">select * from employee
</select><!--    <resultMap id="baseMap" type="employee">-->
<!--        <id column="id" property="id"/>-->
<!--        <result column="username" property="username"/>-->
<!--        <result column="password" property="password"/>-->
<!--        <result column="email" property="email"/>-->
<!--        <result column="age" property="age"/>-->
<!--        <result column="admin" property="admin"/>-->
<!--        &lt;!&ndash;-->
<!--            association用来封装对象属性的-->
<!--            property 对象属性的名称（module层外键的那个属性 Employee的外键属性）-->
<!--            javaType 对象属性的全限定名 (外键属性的全限定名)-->
<!--        &ndash;&gt;-->
<!--        &lt;!&ndash;     <association property="department" javaType="com.bjpowernode.domain.Department">&ndash;&gt;-->
<!--        &lt;!&ndash;         <id column="d_id" property="id"/>&ndash;&gt;-->
<!--        &lt;!&ndash;         <result column="d_name" property="name"/>&ndash;&gt;-->
<!--        &lt;!&ndash;     </association>&ndash;&gt;--><!--        <association property="department" javaType="com.bjpowernode.domain.Department" columnPrefix="d_">-->
<!--            <id column="id" property="id"/>-->
<!--            <result column="name" property="name"/>-->
<!--        </association>-->
<!--    </resultMap>--><resultMap id="baseMap" type="employee"><id column="id" property="id"/><result column="username" property="username"/><result column="password" property="password"/><result column="email" property="email"/><result column="age" property="age"/><result column="admin" property="admin"/><result column="d_id" property="department.id"/><result column="d_name" property="department.name"/></resultMap><select id="selectEmpWithDepart" resultMap="baseMap">
select emp.id,emp.name,emp.username,emp.age,emp.admin, emp.email,emp.`password`,
dept.id d_id, dept.name d_name
from  employee emp left join department dept on emp.dept_id = dept.id
</select><resultMap id="baseMap2" type="employee"><id column="id" property="id" /><result column="username" property="username"/><result column="name" property="name"/><result column="password" property="password"/><result column="email" property="email"/><result column="age" property="age"/><result column="admin" property="admin"/><!--column 属性 它是发送 额外sql的  条件select 属性 它是指定发送额外sql的方法--><association property="dept" column="dept_id"select="com.bjpowernode.mapper.DepartmentMapper.selectDeptById"/></resultMap><select id="selectEmpWithDepart2" resultMap="baseMap2">select * from employee
</select><resultMap id="baseMap3" type="employee"><id column="id" property="id" /><result column="username" property="username"/><result column="name" property="name"/><result column="password" property="password"/><result column="email" property="email"/><result column="age" property="age"/><result column="admin" property="admin"/><!--collection  封装集合类型的属性property 表示 集合类型的属性的名称ofType 表示集合中存储的对象的类型select 表示 指定发送的额外sqlcolumn  表示 执行额外sql 需要的条件--><collection property="roles" ofType="com.bjpowernode.domain.Role"select="com.bjpowernode.mapper.RoleMapper.selectRolesByEmpId"column="id"/></resultMap><select id="selectEmpWithRoles" resultMap="baseMap3">
select * from employee
</select></mapper>
2.8.3 RoleMapper.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.bjpowernode.mapper.RoleMapper"><select id="selectAll" resultType="role">select * from role
</select><select id="selectRolesByEmpId" resultType="role">
select r.* from role r join employee_role er on r.id = er.role_id
where er.employee_id = #{empId}
</select></mapper>

2.9 测试类

package com.bjpowernode.test;import com.bjpowernode.domain.Department;
import com.bjpowernode.domain.Employee;
import com.bjpowernode.mapper.DepartmentMapper;
import com.bjpowernode.mapper.EmployeeMapper;
import com.bjpowernode.util.MyBatisUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;import java.util.List;public class DemoTest {// 需求： 查询所有的部门信息@Testpublic  void testSelectAll(){SqlSession sqlSession = MyBatisUtil.openSession();DepartmentMapper mapper = sqlSession.getMapper(DepartmentMapper.class);List<Department> departments = mapper.selectAll();departments.forEach(System.out::println);sqlSession.close();}// 需求： 查询所有的员工信息@Testpublic  void testSelectAll1(){SqlSession sqlSession = MyBatisUtil.openSession();EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);List<Employee> employees = mapper.selectAll();employees.forEach(System.out::println);sqlSession.close();}//需求: 查询员工信息以及对应的部门信息 （员工信息和对应的部门信心 都要展示）@Testpublic  void testSelectEmpsWithDepart(){SqlSession sqlSession = MyBatisUtil.openSession();EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);List<Employee> employees = mapper.selectEmpWithDepart();employees.forEach(System.out::println);sqlSession.close();}@Testpublic  void testSelectEmpsWithDepart2(){SqlSession sqlSession = MyBatisUtil.openSession();EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);List<Employee> employees = mapper.selectEmpWithDepart2();employees.forEach(System.out::println);sqlSession.close();}// 需求：查询员工信息以及拥有的角色信息（员工信息 要展示）@Testpublic  void testSelectEmpsWithRoles(){SqlSession sqlSession = MyBatisUtil.openSession();EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);List<Employee> employees = mapper.selectEmpWithRoles();for (Employee employee : employees) {System.out.println(employee.getRoles());}sqlSession.close();}/***  完成上述需求：有两种方案 ，一种是 关联查询，一种是 额外sql*  特点： 关联查询  只发送一条sql ，关联了多张表*        额外sql   发送 N + 1 条sql ， 没有关联表*  优缺点：**      如果需求中，查询员工以及部门信息，并要求展示在列表上，那么*      关联查询 方案比较好，而额外sql 方案 不可取，因为发送sql 太多，*      给 sql 服务器带来压力**      如果需求中，查询员工信息以及部门信息，但是 要求展示员工信息，*      不需要展示部门信息  ，那么这个时候 应当使用 额外sql ，而放弃关联查询*      因为额外sql 可以*      配置懒加载模式（需要的时候，发送额外sql，不需要的时候，不发送），*      放弃关联查询 是由于关联表多 ，导致性能低****/
}

3. 过程知识点细讲

3.1 一对多封装 （一个员工对应一个部门）

Employee.java

package com.bjpowernode.domain;import lombok.Data;@Data
public class Employee {private Long id;private String username;private String password;private String email;private Integer age;private boolean admin;private Department department;// 所属部门}

方案一:关联查询（association、property、javaType、[columnPrefix]）

EmployeeMapper.java

 List<Employee> selectAllEmployeeWithDepartment();

EmployeeMapper.xml

association用来封装对象属性的
property 对象属性的名称（module层外键的那个属性 Employee的外键属性）
javaType 对象属性的全限定名 (外键属性的全限定名)

<resultMap id="baseMap" type="employee"><id column="id" property="id"/><result column="username" property="username" /><result column="password" property="password" /><result column="email" property="email" /><result column="age" property="age" /><result column="admin" property="admin" />
<!--association用来封装对象属性的property 对象属性的名称（module层外键的那个属性 Employee的外键属性）javaType 对象属性的全限定名 (外键属性的全限定名)
--><association property="department" javaType="com.bjpowernode.domain.Department"><id column="d_id" property="id"/><result column="d_name" property="name"/></association></resultMap>
columnPrefix

用了columnPrefix属性

<resultMap id="baseMap" type="employee"><id column="id" property="id"/><result column="username" property="username" /><result column="password" property="password" /><result column="email" property="email" /><result column="age" property="age" /><result column="admin" property="admin" />
<!--association用来封装对象属性的property 对象属性的名称（module层外键的那个属性 Employee的外键属性）javaType 对象属性的全限定名 (外键属性的全限定名)
--><association property="department" javaType="com.bjpowernode.domain.Department" columnPrefix="d_"><id column="id" property="id"/><result column="name" property="name"/></association></resultMap>

    <select id="selectAllEmployeeWithDepartment" resultMap="baseMap">
SELECTemp.id,emp.username,emp.age,emp.admin,emp.email,emp.`password`,dept.id d_id,dept.NAME d_name
FROMemployee empLEFT JOIN department dept ON emp.dept_id = dept.id</select>

方案二：发送额外sql（1）（推荐使用这种）

EmployeeMapper.java

   // 发送额外SQLList<Employee> selectAllEmployeeWithDepartmentBySendExtraSQL();

EmployeeMapper.xml

resultMap

column 属性 它是发送额外sql的条件
select 属性 它是指定发送额外sql的方法

<resultMap id="baseMapBySendExtraSQL" type="employee"><id column="id" property="id"/><result column="username" property="username" /><result column="password" property="password" /><result column="email" property="email" /><result column="age" property="age" /><result column="admin" property="admin" /><!--column 属性 它是发送额外sql的条件select 属性 它是指定发送额外sql的方法--><association property="department" column="dept_id"select="com.bjpowernode.mapper.DepartmentMapper.selectDepartmentById"></association></resultMap>

select标签

<select id="selectAllEmployeeWithDepartmentBySendExtraSQL" resultMap="baseMapBySendExtraSQL">select * from employee;</select>

DepartmentMapper.java

  // 根据id 查询部门Department selectDepartmentById();

DepartmentMapper.xml

 <select id="selectDepartmentById" resultType="department">select * from department where id = #{id}</select>

方案二：发送额外sql（2）

EmployeeMapper.java

  // 发送额外SQL TeduList<Employee> selectAllEmployeeWithDepartmentBySendExtraSQLTedu();

EmployeeMapper.xml

   <resultMap id="baseMapBySendExtraSQLTedu" type="employee"><id column="id" property="id"/><result column="username" property="username"/><result column="password" property="password"/><result column="email" property="email"/><result column="age" property="age"/><result column="admin" property="admin"/><!--column 属性 它是发送额外sql的条件select 属性 它是指定发送额外sql的方法--><association property="department" column="dept_id"select="selectDepartmentById"></association></resultMap><select id="selectDepartmentById" resultType="department">select * from department where id = #{id}</select><select id="selectAllEmployeeWithDepartmentBySendExtraSQLTedu" resultMap="baseMapBySendExtraSQLTedu">select * from employee;</select>

测试类

   @Testpublic void selectAllEmployeeWithDepartmentBySendExtraSQLTedu(){SqlSession sqlSession = MyBatisUtil.openSession();EmployeeMapper employeeMapper = sqlSession.getMapper(EmployeeMapper.class);List<Employee> employeeList = employeeMapper.selectAllEmployeeWithDepartmentBySendExtraSQLTedu();employeeList.forEach(System.out::println);sqlSession.close();}

两种方案的对比

/**
* 完成下述需求：有两种方案 ，一种是 关联查询，一种是 额外sql
* 特点： 关联查询 只发送一条sql ，关联了多张表
* 额外sql 发送 N + 1 条sql ， 没有关联表
* 优缺点：
*
* 如果需求中，查询员工以及部门信息，并要求展示在列表上，那么
* 关联查询 方案比较好，而额外sql 方案 不可取，因为发送sql 太多，
* 给 sql 服务器带来压力
*
* 如果需求中，查询员工信息以及部门信息，但是 要求展示员工信息，
* 不需要展示部门信息 ，那么这个时候 应当使用 额外sql ，而放弃关联查询
* 因为额外sql 可以
* 配置懒加载模式（需要的时候，发送额外sql，不需要的时候，不发送），
* 放弃关联查询 是由于关联表多 ，导致性能低
*
*/

3.2 多对多封装（员工和角色）

domain
Role.java

package com.bjpowernode.domain;import lombok.Data;@Data
public class Role {private Long id;private String name;private String sn;
}

Employee.java表


```bash
package com.bjpowernode.domain;import lombok.Data;import java.util.*;@Data
public class Employee {private Long id;private String username;private String password;private String email;private Integer age;private boolean admin;private Department department;// 所属部门private List<Role> roleList = new ArrayList<>();}

RoleMapper.java

package com.bjpowernode.mapper;import com.bjpowernode.domain.Department;
import com.bjpowernode.domain.Role;import java.util.List;public interface RoleMapper {List<Role> selectAllRole();
}

RoleMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.bjpowernode.mapper.RoleMapper"><select id="selectAllRole" resultType="role">select * from role;</select>
</mapper>

mybatis-config.xml

 <!-- 挂载sql的映射文件   --><mappers><mapper resource="com.bjpowernode.mapper\DepartmentMapper.xml"/><mapper resource="com.bjpowernode.mapper\EmployeeMapper.xml"/><mapper resource="com.bjpowernode.mapper\RoleMapper.xml"/></mappers>

测试类

@Test
public void selectAllRole(){SqlSession sqlSession = MyBatisUtil.openSession();RoleMapper roleMapper = sqlSession.getMapper(RoleMapper.class);List<Role> roleList = roleMapper.selectAllRole();roleList.forEach(System.out::println);sqlSession.close();
}

EmployeeMapper.java

// 发送额外SQL
List<Employee> selectAllEmployeeWithRole();

方案一：发送额外SQL （1）（推荐使用这种方式）

EmployeeMapper.xml

<resultMap id="EmpRoleMapBySendExtraSQL" type="employee"><id column="id" property="id"/><result column="username" property="username" /><result column="password" property="password" /><result column="email" property="email" /><result column="age" property="age" /><result column="admin" property="admin" /><!--collection 封装集合类型的属性property 表示 集合类型的属性的名称（module层外键集合属性的名称）ofType 表示集合中存储的对象的类型select 表示 指定发送的额外sqlcolumn 表示 执行额外sql 需要的条件--><collection property="roleList" ofType="com.bjpowernode.domain.Role"select="com.bjpowernode.mapper.RoleMapper.selectRolesByEmpId" column="id"></collection></resultMap><select id="selectAllEmployeeWithRole" resultMap="EmpRoleMapBySendExtraSQL">select * from employee;</select>

RoleMapper.java

   List<Role> selectRolesByEmpId(Long empId);

RoleMapper.xml

   <select id="selectRolesByEmpId" resultType="role">select r.* from role r join employee_role er on r.id = er.role_idwhere er.employee_id = #{empId}
</select>

测试类

  // 需求：查询员工信息以及拥有的角色信息（员工信息要展示）@Testpublic void selectAllEmployeeWithRole(){SqlSession sqlSession = MyBatisUtil.openSession();EmployeeMapper employeeMapper = sqlSession.getMapper(EmployeeMapper.class);List<Employee> employeeList = employeeMapper.selectAllEmployeeWithRole();employeeList.forEach(System.out::println);sqlSession.close();}

发送了额外sql

是否发送额外SQL 配置懒加载

如何让它不发额外SQL呢？配置一个懒加载
将以下代码圈出来的代码复制到mybatis-config.xml文件中

<!-- 设置为 false 表示每个延迟加载属性会按需加载 --><setting name="aggressiveLazyLoading" value="false" /><!--是否开启懒加载 总开关 延迟加载的全局开关--><setting name="lazyLoadingEnabled" value="true"/><!--指定对象的哪些方法触发一次延迟加载--><setting name="lazyLoadTriggerMethods" value="clone"/>

修改Employee.java
@Data改为@Getter和@Setter
重写toString方法（不重写roleList）

package com.bjpowernode.domain;import lombok.Data;
import lombok.Getter;
import lombok.Setter;import java.util.*;@Getter
@Setter
public class Employee {private Long id;private String username;private String password;private String email;private Integer age;private boolean admin;private Department department;// 所属部门private List<Role> roleList = new ArrayList<>();@Overridepublic String toString() {return "Employee{" +"id=" + id +", username='" + username + '\'' +", password='" + password + '\'' +", email='" + email + '\'' +", age=" + age +", admin=" + admin +", department=" + department +'}';}
}

再次执行测试类
就没有发送额外SQL了

测试类
当测试类调用getRoleList的时候，就又会发送额外SQL了

 // 需求：查询员工信息以及拥有的角色信息（员工信息要展示）@Testpublic void selectAllEmployeeWithRole(){SqlSession sqlSession = MyBatisUtil.openSession();EmployeeMapper employeeMapper = sqlSession.getMapper(EmployeeMapper.class);List<Employee> employeeList = employeeMapper.selectAllEmployeeWithRole();for (Employee employee : employeeList) {System.out.println(employee.getRoleList());}sqlSession.close();}

又会发送额外SQL了

方案一：发送额外SQL（2）

EmployeeMapper.java

// 发送额外SQL TeduList<Employee> selectAllEmployeeWithRoleBySendExtraSQLTedu();

EmployeeMapper.xml

 <resultMap id="EmpRoleMapBySendExtraSQLTedu" type="employee"><id column="id" property="id"/><result column="username" property="username"/><result column="password" property="password"/><result column="email" property="email"/><result column="age" property="age"/><result column="admin" property="admin"/><!--collection 封装集合类型的属性property 表示 集合类型的属性的名称（module层外键 集合属性的名称）ofType 表示集合中存储的对象的类型select 表示 指定发送的额外sql接口权限定名column 表示 执行额外sql 需要的条件--><collection property="roleList" ofType="com.bjpowernode.domain.Role"select="selectRolesByEmpId" column="id"></collection></resultMap><select id="selectRolesByEmpId" resultType="role">select r.* from role r join employee_role er on r.id = er.role_idwhere er.employee_id = #{empId}</select><select id="selectAllEmployeeWithRoleBySendExtraSQLTedu" resultMap="EmpRoleMapBySendExtraSQLTedu">select * from employee;</select>

测试类

 @Testpublic void selectAllEmployeeWithRoleBySendExtraSQuLTedu(){SqlSession sqlSession = MyBatisUtil.openSession();EmployeeMapper employeeMapper = sqlSession.getMapper(EmployeeMapper.class);List<Employee> employeeList = employeeMapper.selectAllEmployeeWithRoleBySendExtraSQLTedu();for (Employee employee : employeeList) {System.out.println(employee.getRoleList());}sqlSession.close();}

方案二：关联查询

EmployeeMapper.java

   //关联查询List<Employee> selectAllEmployeeWithRole();

EmployeeMapper.xml

<resultMap id="EmpRoleMap" type="employee"><id column="id" property="id"/><result column="username" property="username"/><result column="password" property="password"/><result column="email" property="email"/><result column="age" property="age"/><result column="admin" property="admin"/><!--collection 封装集合类型的属性property 表示 集合类型的属性的名称（module层外键 集合属性的名称）ofType 表示集合中存储的对象的类型select 表示 指定发送的额外sql接口权限定名column 表示 执行额外sql 需要的条件--><collection property="roleList" ofType="role" columnPrefix="r_"><id column="id" property="id"/><result column="name" property="name"/></collection></resultMap><select id="selectAllEmployeeWithRole" resultMap="EmpRoleMap">
SELECTemp.*,r.id r_id,r.NAME r_name
FROMemployee empJOIN employee_role er ON emp.id = er.employee_idJOIN role r ON r.id = er.role_id</select>

Emloyee.java

package com.bjpowernode.domain;import lombok.Data;
import lombok.Getter;
import lombok.Setter;
import lombok.ToString;import java.util.*;@Getter
@Setter
@ToString
public class Employee {private Long id;private String username;private String password;private String email;private Integer age;private boolean admin;private Department department;// 所属部门private List<Role> roleList = new ArrayList<>();//    @Override
//    public String toString() {//        return "Employee{" +
//                "id=" + id +
//                ", username='" + username + '\'' +
//                ", password='" + password + '\'' +
//                ", email='" + email + '\'' +
//                ", age=" + age +
//                ", admin=" + admin +
//                ", department=" + department +
//                '}';
//    }
}

测试类

@Test
public void selectAllEmployeeWithRole(){SqlSession sqlSession = MyBatisUtil.openSession();EmployeeMapper employeeMapper = sqlSession.getMapper(EmployeeMapper.class);List<Employee> employeeList = employeeMapper.selectAllEmployeeWithRole();for (Employee employee : employeeList) {System.out.println(employee);}sqlSession.close();
}

总结

配置文件中 两个标签 ：association collection 的选择
首先 标签 和 使用的查询方案 没有任何关系
如果类中有对象属性，则使用association标签
如果类中有集合对象属性，则使用 collection

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