关联关系
在关系型数据库中，表与表之间很少是独立且没有关系的，大多数表都具有复杂的关系，一张表连接着多张表，在MyBatis中可以进行多表连接处理
关联关系中有1对1、1对多、多对多
1对1关系
我们有一张员工表(t_emp)，一张部门表(t_dept)。员工表中的一条记录对应于部门表中有且仅有一条记录。这就是一对一的关联关系。
查询每个员工的信息及对应的部门信息
1、创建maven项目
在pom.xml文件中导入相关依赖
<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.zhouym</groupId>
<artifactId>mybatis-lazy</artifactId>
<version>0.0.1-SNAPSHOT</version>
<dependencies>
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.43</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-log4j12</artifactId>
<version>1.7.25</version>
</dependency>
<dependency>
<groupId>org.mybatis</groupId>
<artifactId>mybatis</artifactId>
<version>3.4.6</version>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.12</version>
</dependency>
</dependencies>
</project>
2、导入相关资源文件
db.properties
driverName=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/tb_user?useSSL=true&useUnicode=true&characterEncoding=UTF-8
userName=root
password=123456
log4j.properties
log4j.rootCategory=DEBUG, stdout , R
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=[QC] %p [%t] %C.%M(%L) | %m%n
log4j.appender.R=org.apache.log4j.DailyRollingFileAppender
log4j.appender.R.File=E:\\tools\\log\\qc.log
log4j.appender.R.layout=org.apache.log4j.PatternLayout
log4j.appender.R.layout.ConversionPattern=%d-[TS] %p %t %c - %m%n
mybatis配置文件
<?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>
<!--引入数据库配置信息-->
<properties resource="db.properties" />
<typeAliases>
<!-- typeAlias 具体的给每一个pojo类取别名，如果pojo太多，取名很麻烦 -->
<!-- <typeAlias type="com.zhouym.pojo.User" alias="user"/> -->
<!-- 指定一个基础路径 不区分大小写 -->
<package name="com.zhouym.pojo" />
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC" />
<dataSource type="POOLED">
<property name="driver" value="${driverName}" />
<property name="url" value="${url}" />
<property name="username" value="${userName}" />
<property name="password" value="${password}" />
</dataSource>
</environment>
</environments>
<!-- 关联映射文件 -->
<mappers>
<!-- package 在接口的方式下使用，接口名称必须和映射文件名称一致 -->
<package name="com.zhouym.dao" />
</mappers>
</configuration>
3、POJO类
对应数据库中的表
员工表
package com.zhouym.pojo;
public class Emp {
private Integer id;
private String emp_name;
private Integer dept_id;
private Dept dept;
public Emp() {
super();
}
public Emp(Integer id, String emp_name, Integer dept_id) {
super();
this.id = id;
this.emp_name = emp_name;
this.dept_id = dept_id;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getEmp_name() {
return emp_name;
}
public void setEmp_name(String emp_name) {
this.emp_name = emp_name;
}
public Integer getDept_id() {
return dept_id;
}
public void setDept_id(Integer dept_id) {
this.dept_id = dept_id;
}
public Dept getDept() {
return dept;
}
public void setDept(Dept dept) {
this.dept = dept;
}
@Override
public String toString() {
return "Emp [id=" + id + ", emp_name=" + emp_name + ", dept_id=" + dept_id + "]";
}
}
4、接口类及对应的映射文件
package com.zhouym.dao;
import java.util.List;
import com.zhouym.pojo.Emp;
public interface EmpMapper {
public List<Emp> query();
}
<?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.zhouym.dao.EmpMapper">
<!--因为在mybatis配置文件中配置了包名路径，package=“com.zhouym.pojo”,在type中直接写对应的类，不区分大小写-->
<resultMap type="emp" id="baseMap">
<id property="id" column="id"/>
<result property="emp_name" column="emp_name"/>
<result property="dept_id" column="dept_id"/>
<!-- 一对一关系 -->
<association property="dept" javaType="dept">
<id property="id" column="did"/>
<result property="dept_name" column="dept_name"/>
<result property="dept_desc" column="dept_desc"/>
</association>
</resultMap>
<select id="query" resultMap="baseMap">
select
t1.id,
t1.emp_name,
t1.dept_id,
t2.id did,
t2.dept_name,
t2.dept_desc
from
t_emp t1 left join t_dept t2
on t1.dept_id = t2.id
</select>
</mapper>
5、工具类
DBUtil
package com.zhouym.DBUtil;
import java.io.IOException;
import java.io.InputStream;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
public class DBUtils {
private static InputStream in;
private static SqlSessionFactory factory = null;
public static SqlSessionFactory getSqlSessionFactory() throws IOException {
if (factory == null) {
in = Resources.getResourceAsStream("mybatis-cfg.xml");
factory = new SqlSessionFactoryBuilder().build(in);
}
return factory;
}
}
6、测试类
package com.zhouym.junit;
import java.io.IOException;
import java.util.List;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.junit.Test;
import com.zhouym.DBUtil.DBUtils;
import com.zhouym.dao.DeptMapper;
import com.zhouym.dao.EmpMapper;
import com.zhouym.pojo.Dept;
import com.zhouym.pojo.Emp;
public class JunitTest {
@Test
public void test() throws IOException {
SqlSessionFactory factory = DBUtils.getSqlSessionFactory();
SqlSession session = factory.openSession();
EmpMapper mapper = session.getMapper(EmpMapper.class);
List<Emp> list = mapper.query();
for (Emp emp : list) {
System.out.println(emp);
System.out.println("===>"+emp.getDept());
}
}
}
一对一关系测试结果
1对多关系
增加部门表
package com.zhouym.pojo;
import java.util.List;
public class Dept {
private Integer id;
private String dept_name;
private String dept_desc;
private List<Emp> emps;
@Override
public String toString() {
return "Dept [id=" + id + ", dept_name=" + dept_name + ", dept_desc=" + dept_desc +"]";
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getDept_name() {
return dept_name;
}
public void setDept_name(String dept_name) {
this.dept_name = dept_name;
}
public String getDept_desc() {
return dept_desc;
}
public void setDept_desc(String dept_desc) {
this.dept_desc = dept_desc;
}
public List<Emp> getEmps() {
return emps;
}
public void setEmps(List<Emp> emps) {
this.emps = emps;
}
}
部门接口类及映射文件
package com.zhouym.dao;
import java.util.List;
import com.zhouym.pojo.Dept;
public interface DeptMapper {
public List<Dept> query();
}
<?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">
<!-- 部门与员工是1对多关系 -->
<mapper namespace="com.zhouym.dao.DeptMapper">
<resultMap type="dept" id="baseMap">
<id property="id" column="id"/>
<result property="dept_name" column="dept_name"/>
<result property="dept_desc" column="dept_desc"/>
<collection property="emps" ofType="emp">
<id property="id" column="eid"/>
<result property="emp_name" column="emp_name"/>
<result property="dept_id" column="dept_id"/>
</collection>
</resultMap>
<select id="query" resultMap="baseMap">
select
t1.id,
t1.dept_name,
t1.dept_desc,
t2.id eid,
t2.emp_name,
t2.dept_id
from
t_dept t1
left join
t_emp t2
on
t1.id = t2.dept_id
</select>
</mapper>
其他按上面的配置
测试类
package com.zhouym.junit;
import java.io.IOException;
import java.util.List;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.junit.Test;
import com.zhouym.DBUtil.DBUtils;
import com.zhouym.dao.DeptMapper;
import com.zhouym.dao.EmpMapper;
import com.zhouym.pojo.Dept;
import com.zhouym.pojo.Emp;
public class JunitTest {
@Test
public void test1() throws IOException {
SqlSessionFactory factory = DBUtils.getSqlSessionFactory();
SqlSession session = factory.openSession();
DeptMapper mapper = session.getMapper(DeptMapper.class);
List<Dept> list = mapper.query();
for (Dept dept : list) {
System.out.println(dept.getDept_name()+"--->"+dept.getEmps());
}
}
}
测试结果
多对多关系
双向的一对多也是多对多关系
来源：CSDN
作者：zhouym_
链接：https://blog.csdn.net/zhouym_/article/details/95982618