mybatis关联查询之一对多，多对一，以及多对多

一、使用IDEA新建maven工程

二、引入mybatis以及相关的jar

<!--版本仅供参考-->
<dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
        </dependency>
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.4.5</version>
        </dependency>
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.17</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.44</version>
        </dependency>
    </dependencies>

三、建表

　　权限管理需求的介绍，一个用户拥有若干角色，一个角色拥有若干权限，权限就是对某个资源的增删改查，这样就构成了用户-角色-权限之间的模型，在这种关系中，用户与角色，角色与权限之间一般是多对多的关系

　建表sql：

create table sys_user(
    id int not null auto_increment comment '用户ID',
    user_name varchar(32) comment '用户名',
    password varchar(50) comment '用户密码',
    usr_info varchar(50) comment '用户简介',
    head_img blob comment '头像',
    create_time datetime comment '创建时间',
    primary key(id)
);
alter table sys_user comment  '用户表';

create table sys_role(
    id int not null auto_increment comment  '角色ID',
    role_name varchar(50) comment '角色名称',
    enabled  int comment '有效标志',
    create_by  bigint comment '创建人',
    create_time datetime comment '创建时间',
    primary key(id)    
);
alter table sys_role comment '角色表';

create table sys_privilege(
    id int not null auto_increment  comment '权限ID',
    privilege_name varchar(50) comment '权限名称',
    privilege_url varchar(50) comment '权限URL',
    primary key(id)
);
alter table sys_privilege comment '权限表';

create table sys_user_role(
    user_id bigint not null comment '用户ID',
    role_id bigint not null comment '角色ID'
);
alter table sys_user comment '用户角色';
create table sys_role_privilege(
    role_id bigint not null comment '角色ID',
    privilege_id  bigint  not null comment '权限ID'
);
alter table sys_role_privilege comment '角色权限表';

给表添加外键

--   给用户角色表的user_id添加外键 
alter table sys_user_role add constraint fk_uru foreign key(user_id) references sys_user(id);
--   给用户角色表的role_id 添加外键 
alter table sys_user_role add constraint fk_sysrur foreign key(role_id) references sys_role(id);
--    给角色权限表的role_id添加外键 
alter table sys_role_privilege add constraint fk_sysrpp foreign key(privilege_id) references sys_privilege(id);
--    给角权限的privilege_id 添加外键 
alter table sys_role_privilege add constraint fk_sysrpr foreign key(role_id) references sys_role(role_id);

插入数据

insert into sys_user(user_name,password,user_info,head_img,create_time )values('Jordan','123','管理员',null,'2018-1-22 12:02:01'),
('Alice','456','测试人员',null,'2018-1-22 12:02:01'),
('Alex','789','开发人员',null,'2018-1-22 12:02:01'),
('James','012','运维人员',null,'2018-1-22 12:02:01')
insert into sys_user_role(role_name,enabled,create_by,create_time) values('管理员',1,'1','2018-01-02'),values('普通用户',1,'1','2018-01-02');
insert into sys_privilege(privilege_name,privilege_url) values('用户管理','/users'),('角色管理','/roles'),('系统维护','/system'),('日志管理','/logs');
insert into sys_user_role values(1,1),(2,2),(3,2),(4,2);
insert into sys_role_privilege values(1,1),(2,3),(2,4);

四、使用逆向工程生成entity以及mapper接口和mapper文件

　　①：创建mybatis-config.xml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>
    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/>
    </settings>
    <typeAliases>
        <package name="com.jordan.mybatis.entity"></package>
        </typeAliases>
    <environments default="defaultEnv">
        <environment >
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="username" value="root"></property>
                <property name="password" value="123456"></property>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis"></property>
                <property name="driver" value="com.mysql.jdbc.Driver"></property>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <package name="com.jordan.mybatis.mapper"></package>
    </mappers>
</configuration>

　　②：SysUser.java

package com.jordan.mybatis.entity;

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

public class SysUser {
    private Long id;

    private String userName;

    private String password;

    private Date createTime;

    private String userInfo;

    private byte[] headImg;

    private SysRole sysRole;

    private List<SysRole> sysRoleList;

    public List<SysRole> getSysRoleList() {
        return sysRoleList;
    }

    public void setSysRoleList(List<SysRole> sysRoleList) {
        this.sysRoleList = sysRoleList;
    }

    public SysRole getSysRole() {
        return sysRole;
    }

    public void setSysRole(SysRole sysRole) {
        this.sysRole = sysRole;
    }

    public Long getId() {
        return id;
    }

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

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName == null ? null : userName.trim();
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password == null ? null : password.trim();
    }

    public Date getCreateTime() {
        return createTime;
    }

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


    public String getUserInfo() {
        return userInfo;
    }

    public void setUserInfo(String userInfo) {
        this.userInfo = userInfo == null ? null : userInfo.trim();
    }

    public byte[] getHeadImg() {
        return headImg;
    }

    public void setHeadImg(byte[] headImg) {
        this.headImg = headImg;
    }

    @Override
    public String toString() {
        return "SysUser{" +
                "id=" + id +
                ", userName='" + userName + '\'' +
                ", password='" + password + '\'' +
                ", createTime=" + createTime +
                ", userInfo='" + userInfo + '\'' +
                ", headImg=" + Arrays.toString(headImg) +
                ", sysRole=" + sysRole +
                ", sysRoleList=" + sysRoleList +
                '}';
    }
}

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