mybatis中的多表查询：
示例：用户和角色
一个用户可以有多个角色
一个角色可以赋予多个用户
步骤：
1、建立两张表：用户表，角色表
让用户表和角色表具有多对多的关系。需要使用中间表，中间表中包含各自的主键，在中间表中是外键。
2、建立两个实体类：用户实体类和角色实体类
让用户和角色的实体类能体现出来多对多的关系
各自包含对方一个集合引用
3、建立两个配置文件
用户的配置文件
角色的配置文件
4、实现配置：
当我们查询用户时，可以同时得到用户所包含的角色信息
当我们查询角色时，可以同时得到角色的所赋予的用户信息
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业务要求多对多关系其实我们看成是双向的一对多关系。
用户与角色的关系模型需求：当我们查询角色时，可以同时得到角色的所赋予的用户信息。分析：查询角色我们需要用到Role表，但角色分配的用户的信息我们并不能直接找到用户信息，而是要通过中间表(USER_ROLE 表)才能关联到用户信息。
用户与角色的多对多关系模型如下：
角色表：
用户表：
用户角色中间表：
Role：
package com.keafmd.domain;
import java.io.Serializable;
import java.util.List;
/**
* Keafmd
*
* @ClassName: Role
* @Description: 角色实体类
* @author: 牛哄哄的柯南
* @date: 2021-02-12 16:45
*/
public class Role implements Serializable {
private Integer roleId;
private String roleName;
private String roleDesc;
//多对多的关系映射
private List<User> users;
public List<User> getUsers() {
return users;
}
public void setUsers(List<User> users) {
this.users = users;
}
public Integer getRoleId() {
return roleId;
}
public void setRoleId(Integer roleId) {
this.roleId = roleId;
}
public String getRoleName() {
return roleName;
}
public void setRoleName(String roleName) {
this.roleName = roleName;
}
public String getRoleDesc() {
return roleDesc;
}
public void setRoleDesc(String roleDesc) {
this.roleDesc = roleDesc;
}
@Override
public String toString() {
return "Role{"
"roleId=" roleId
", roleName='" roleName '\''
", roleDesc='" roleDesc '\''
'}';
}
}
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IRoledao：
package com.keafmd.dao;
import com.keafmd.domain.Role;
import java.util.List;
/**
* Keafmd
*
* @ClassName: IRoleDao
* @Description:
* @author: 牛哄哄的柯南
* @date: 2021-02-12 19:01
*/
public interface IRoleDao {
/**
* 查询所有角色
* @return
*/
List<Role> findAll();
}
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select u.*,r.id as rid ,r.role_name ,r.role_desc from role r
left outer join user_role ur on r.id = ur.rid
left outer join user u on u.id=ur.uid
123
执行结果：
编写映射文件注意：sql语句换行的时候最好在每行的末尾或开头添加空格，这样可以防止合并成一行时发生错误。
IRoleDao.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.keafmd.dao.IRoleDao">
<!--定义role表的resultmap-->
<resultMap id="roleMap" type="role">
<id property="roleId" column="rid"></id>
<result property="roleName" column="role_name"></result>
<result property="roleDesc" column="role_desc"></result>
<collection property="users" ofType="user">
<id column="id" property="id"></id>
<result column="username" property="username"></result>
<result column="address" property="address"></result>
<result column="sex" property="sex"></result>
<result column="birthday" property="birthday"></result>
</collection>
</resultMap>
<!--查询所有-->
<select id="findAll" resultMap="roleMap">
select u.*,r.id as rid ,r.role_name ,r.role_desc from role r
left outer join user_role ur on r.id = ur.rid
left outer join user u on u.id=ur.uid
</select>
</mapper>
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RoleTest：
package com.keafmd.test;
import com.keafmd.dao.IRoleDao;
import com.keafmd.dao.IUserDao;
import com.keafmd.domain.Role;
import com.keafmd.domain.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.After;
import org.junit.Before;
import org.junit.Test;
import java.io.InputStream;
import java.util.List;
/**
* Keafmd
*
* @ClassName: MybatisTest
* @Description: 测试类，测试crud操作
* @author: 牛哄哄的柯南
* @date: 2021-02-08 15:24
*/
public class RoleTest {
private InputStream in;
private SqlSession sqlsession;
private IRoleDao roleDao;
@Before // 用于在测试方法执行前执行
public void init()throws Exception{
//1.读取配置文件,生成字节输入流
in = Resources.getResourceAsStream("SqlMapConfig.xml");
//2.创建SqlSessionFactory工厂
SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
SqlSessionFactory factory = builder.build(in);
//3.使用工厂生产SqlSession对象
sqlsession = factory.openSession(); //里面写个true，下面每次就不用了写 sqlsession.commit(); 了
//4.使用SqlSession创建Dao接口的代理对象
roleDao = sqlsession.getMapper(IRoleDao.class);
}
@After // 用于在测试方法执行后执行
public void destory() throws Exception{
//提交事务
sqlsession.commit();
//6.释放资源
sqlsession.close();
in.close();
}
/**
* 查询所有
* @throws Exception
*/
@Test
public void testFindAll() {
List<Role> roles = roleDao.findAll();
for (Role role : roles) {
System.out.println("--------每个角色的信息---------");
System.out.println(role);
System.out.println(role.getUsers());
}
}
}
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运行结果：
2021-02-13 00:05:47,481 349 [ main] DEBUG ansaction.jdbc.JdbcTransaction - Opening JDBC Connection
2021-02-13 00:05:47,784 652 [ main] DEBUG source.pooled.PooledDataSource - Created connection 1027007693.
2021-02-13 00:05:47,784 652 [ main] DEBUG ansaction.jdbc.JdbcTransaction - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3d36e4cd]
2021-02-13 00:05:47,791 659 [ main] DEBUG om.keafmd.dao.IRoleDao.findAll - ==> Preparing: select u.*,r.id as rid ,r.role_name ,r.role_desc from role r left outer join user_role ur on r.id = ur.rid left outer join user u on u.id=ur.uid
2021-02-13 00:05:47,842 710 [ main] DEBUG om.keafmd.dao.IRoleDao.findAll - ==> Parameters:
2021-02-13 00:05:47,869 737 [ main] DEBUG om.keafmd.dao.IRoleDao.findAll - <== Total: 4
--------每个角色的信息---------
Role{roleId=1, roleName='院长', roleDesc='管理整个学院'}
[User{id=41, username='老王', sex='男', address='北京', birthday=Tue Feb 27 17:47:08 CST 2018}, User{id=45, username='新一', sex='男', address='北京', birthday=Sun Mar 04 12:04:06 CST 2018}]
--------每个角色的信息---------
Role{roleId=2, roleName='总裁', roleDesc='管理整个公司'}
[User{id=41, username='老王', sex='男', address='北京', birthday=Tue Feb 27 17:47:08 CST 2018}]
--------每个角色的信息---------
Role{roleId=3, roleName='校长', roleDesc='管理整个学校'}
[]
2021-02-13 00:05:47,871 739 [ main] DEBUG ansaction.jdbc.JdbcTransaction - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3d36e4cd]
2021-02-13 00:05:47,872 740 [ main] DEBUG ansaction.jdbc.JdbcTransaction - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@3d36e4cd]
2021-02-13 00:05:47,872 740 [ main] DEBUG source.pooled.PooledDataSource - Returned connection 1027007693 to pool.
Process finished with exit code 0
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编写用户实体类需求：当我们查询用户时，可以同时得到用户所包含的角色信息。分析：相比上面的实现 Role 到 User 多对多，主要变化就是sql语句的变化。
User：
package com.keafmd.domain;
import java.io.Serializable;
import java.util.Date;
import java.util.List;
/**
* Keafmd
*
* @ClassName: User
* @Description:
* @author: 牛哄哄的柯南
* @date: 2021-02-08 15:16
*/
public class User implements Serializable {
private Integer id;
private String username;
private String sex;
private String address;
private Date birthday;
//多对多的关系映射，一个用户可以具备多个角色
private List<Role> roles;
public List<Role> getRoles() {
return roles;
}
public void setRoles(List<Role> roles) {
this.roles = roles;
}
public Integer getId() {
return id;
}
public void setId(Integer 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 String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
public Date getBirthday() {
return birthday;
}
public void setBirthday(Date birthday) {
this.birthday = birthday;
}
@Override
public String toString() {
return "User{"
"id=" id
", username='" username '\''
", sex='" sex '\''
", address='" address '\''
", birthday=" birthday
'}';
}
}
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IUserDao：
package com.keafmd.dao;
import com.keafmd.domain.User;
import java.util.List;
/**
* Keafmd
*
* @ClassName: IUserDao
* @Description: 用户的持久层接口
* @author: 牛哄哄的柯南
* @date: 2021-02-06 19:29
*/
public interface IUserDao {
/**
* 查询所有用户，同时获取到用户下所有账户的信息
* @return
*/
List<User> findAll();
}
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select u.*,r.id as rid ,r.role_name ,r.role_desc from user u
left outer join user_role ur on u.id = ur.uid
left outer join role r on r.id=ur.rid
123
执行结果：
IUserDao.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.keafmd.dao.IUserDao">
<!--定义User的resultMap-->
<resultMap id="userMap" type="user">
<id property="id" column="id"></id>
<result property="username" column="username"></result>
<result property="address" column="address"></result>
<result property="sex" column="sex"></result>
<result property="birthday" column="birthday"></result>
<!--配置角色集合的映射-->
<collection property="roles" ofType="role">
<id property="roleId" column="rid"></id>
<result property="roleName" column="role_name"></result>
<result property="roleDesc" column="role_desc"></result>
</collection>
</resultMap>
<!--配置查询所有-->
<select id="findAll" resultMap="userMap">
select u.*,r.id as rid ,r.role_name ,r.role_desc from user u
left outer join user_role ur on u.id = ur.uid
left outer join role r on r.id=ur.rid
</select>
</mapper>
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UserTest ：
package com.keafmd.test;
import com.keafmd.dao.IUserDao;
import com.keafmd.domain.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.After;
import org.junit.Before;
import org.junit.Test;
import java.io.InputStream;
import java.util.List;
/**
* Keafmd
*
* @ClassName: MybatisTest
* @Description: 测试类，测试crud操作
* @author: 牛哄哄的柯南
* @date: 2021-02-08 15:24
*/
public class UserTest {
private InputStream in;
private SqlSession sqlsession;
private IUserDao userDao;
@Before // 用于在测试方法执行前执行
public void init()throws Exception{
//1.读取配置文件,生成字节输入流
in = Resources.getResourceAsStream("SqlMapConfig.xml");
//2.创建SqlSessionFactory工厂
SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
SqlSessionFactory factory = builder.build(in);
//3.使用工厂生产SqlSession对象
sqlsession = factory.openSession(); //里面写个true，下面每次就不用了写 sqlsession.commit(); 了
//4.使用SqlSession创建Dao接口的代理对象
userDao = sqlsession.getMapper(IUserDao.class);
}
@After // 用于在测试方法执行后执行
public void destory() throws Exception{
//提交事务
sqlsession.commit();
//6.释放资源
sqlsession.close();
in.close();
}
/**
* 查询所有
* @throws Exception
*/
@Test
public void testFindAll() {
List<User> users = userDao.findAll();
for (User user : users) {
System.out.println("--------每个用户的信息---------");
System.out.println(user);
System.out.println(user.getRoles());
}
}
}
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运行结果：
2021-02-13 00:17:32,971 422 [ main] DEBUG ansaction.jdbc.JdbcTransaction - Opening JDBC Connection
2021-02-13 00:17:33,275 726 [ main] DEBUG source.pooled.PooledDataSource - Created connection 1027007693.
2021-02-13 00:17:33,275 726 [ main] DEBUG ansaction.jdbc.JdbcTransaction - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3d36e4cd]
2021-02-13 00:17:33,280 731 [ main] DEBUG om.keafmd.dao.IUserDao.findAll - ==> Preparing: select u.*,r.id as rid ,r.role_name ,r.role_desc from user u left outer join user_role ur on u.id = ur.uid left outer join role r on r.id=ur.rid
2021-02-13 00:17:33,319 770 [ main] DEBUG om.keafmd.dao.IUserDao.findAll - ==> Parameters:
2021-02-13 00:17:33,343 794 [ main] DEBUG om.keafmd.dao.IUserDao.findAll - <== Total: 10
--------每个用户的信息---------
User{id=41, username='老王', sex='男', address='北京', birthday=Tue Feb 27 17:47:08 CST 2018}
[Role{roleId=1, roleName='院长', roleDesc='管理整个学院'}, Role{roleId=2, roleName='总裁', roleDesc='管理整个公司'}]
--------每个用户的信息---------
User{id=42, username='update', sex='男', address='XXXXXXX', birthday=Mon Feb 08 19:37:31 CST 2021}
[]
--------每个用户的信息---------
User{id=43, username='小二王', sex='女', address='北京', birthday=Sun Mar 04 11:34:34 CST 2018}
[]
--------每个用户的信息---------
User{id=45, username='新一', sex='男', address='北京', birthday=Sun Mar 04 12:04:06 CST 2018}
[Role{roleId=1, roleName='院长', roleDesc='管理整个学院'}]
--------每个用户的信息---------
User{id=50, username='Keafmd', sex='男', address='XXXXXXX', birthday=Mon Feb 08 15:44:01 CST 2021}
[]
--------每个用户的信息---------
User{id=51, username='update DAO', sex='男', address='XXXXXXX', birthday=Tue Feb 09 11:31:38 CST 2021}
[]
--------每个用户的信息---------
User{id=52, username='Keafmd DAO', sex='男', address='XXXXXXX', birthday=Tue Feb 09 11:29:41 CST 2021}
[]
--------每个用户的信息---------
User{id=53, username='Keafmd laset insertid 1', sex='男', address='XXXXXXX', birthday=Fri Feb 12 20:53:46 CST 2021}
[]
--------每个用户的信息---------
User{id=54, username='Keafmd laset insertid 2 auto', sex='男', address='XXXXXXX', birthday=Fri Feb 12 21:02:12 CST 2021}
[]
2021-02-13 00:17:33,345 796 [ main] DEBUG ansaction.jdbc.JdbcTransaction - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3d36e4cd]
2021-02-13 00:17:33,346 797 [ main] DEBUG ansaction.jdbc.JdbcTransaction - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@3d36e4cd]
2021-02-13 00:17:33,346 797 [ main] DEBUG source.pooled.PooledDataSource - Returned connection 1027007693 to pool.
Process finished with exit code 0
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,以上就是Mybatis的多表关联查询（多对多）的全部内容。
