Mybatis 复习笔记02

本文只用于个人复习mybatis使用：

mybatis中的多表查询，一对一，一对多，多对多

数据库表如下：

user表

account表

role表

1 、编写实体

创建实体：包含一对多，和多对多的实体User

package com.cimo.Entity;
import java.io.Serializable;
import java.util.Date;
import java.util.List;

public class User implements Serializable {
    private Integer id;
    private String username;
    private Date birthday;
    private String sex;
    private String address;

    /**
     * 一对多关系映射：主表实体应该包含从表实体的集合引用
     */
    private List<Account> accounts;

    /**
     * 多对多关系映射：两个实体类都必须包含对方的一个集合引用
     * （多对多关系其实我们看成是双向的一对多关系）
     */
    private List<Role> roles;

    public List<Role> getRoles() {
        return roles;
    }

    public void setRoles(List<Role> roles) {
        this.roles = roles;
    }

    public List<Account> getAccounts() {
        return accounts;
    }

    public void setAccounts(List<Account> accounts) {
        this.accounts = accounts;
    }

    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 Date getBirthday() {
        return birthday;
    }

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }

    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;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", birthday=" + birthday +
                ", sex='" + sex + '\'' +
                ", address='" + address + '\'' +
                ", accounts=" + accounts +
                ", roles=" + roles +
                '}';
    }
}

实体：Account

package com.cimo.Entity;

import java.io.Serializable;

public class Account implements Serializable {

    private Integer id;
    private Integer uid;
    private Double money;

    private User user;

    public Integer getId() {
        return id;
    }

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

    public Integer getUid() {
        return uid;
    }

    public void setUid(Integer uid) {
        this.uid = uid;
    }

    public Double getMoney() {
        return money;
    }

    public void setMoney(Double money) {
        this.money = money;
    }

    public User getUser() {
        return user;
    }

    public void setUser(User user) {
        this.user = user;
    }

    @Override
    public String toString() {
        return "Account{" +
                "id=" + id +
                ", uid=" + uid +
                ", money=" + money +
                ", user=" + user +
                '}';
    }
}

多对多实体：Role

package com.cimo.Entity;

import java.io.Serializable;
import java.util.List;

public class Role implements Serializable {

    private Integer roleId;
    private String roleName;
    private String roleDesc;

    /**
     * 多对多关系映射：两个实体类都必须包含对方的一个集合引用
     * （多对多关系其实我们看成是双向的一对多关系）
     */
    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 + '\'' +
                ", users=" + users +
                '}';
    }
}

2 、编写映射

IAccountDao.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.cimo.DAO.IAccountDao">

    <!-- 定义可以封装带有User的Account的resultMap  -->
    <resultMap id="accountUserMap" type="com.cimo.Entity.Account">
        <id property="id" column="aid"></id>
        <result property="uid" column="uid"></result>
        <result property="money" column="money"></result>
        <!-- 一对一的关系映射，配置封装user的内容 -->
        <association property="user" javaType="com.cimo.Entity.User">
            <id property="id" column="id"></id>
            <result property="username" column="username"></result>
            <result property="birthday" column="birthday"></result>
            <result property="sex" column="sex"></result>
            <result property="address" column="address"></result>
        </association>
    </resultMap>

    <!-- 查询所有 -->
    <select id="findAll" resultMap="accountUserMap">
        SELECT u.*, a.id AS aid, a.uid, a.money from account a, user u WHERE a.uid = u.id
    </select>
</mapper>

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.cimo.DAO.IRoleDao">

    <!-- 配置从用户到角色的多对多roleMap -->
    <resultMap id="roleMap" type="com.cimo.Entity.Role">
        <id property="roleId" column="rid"></id>
        <result property="roleName" column="role_name"></result>
        <result property="roleDesc" column="role_desc"></result>
        <!-- 配置role对象中users集合的映射 -->
        <collection property="users" ofType="com.cimo.Entity.User">
            <id property="id" column="id"/>
            <result property="username" column="username"/>
            <result property="sex" column="sex"/>
            <result property="birthday" column="birthday"/>
            <result property="address" column="address"/>
        </collection>
    </resultMap>

    <!-- 查询所有用户 -->
    <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 ur.uid = u.id;
    </select>
</mapper>

3、运行结果

多对多

4、总结

1. 处理多对多关系时，需要避免在resultMap中出现两个类出现相同的主键，否则会导致相同主键的多对多关系只显示一条数据
2. mybatis_config.xml中可以使用<package>标签以包的方式来添加映射

    <!-- 指定映射配置文件的位置 -->
    <mappers>
        <package name="com.cimo.DAO"/>
    </mappers>

3. resultMap中<association>和<collection>的区别
   <association>用于对单独的类进行映射，使用javaType确定类型
   <collection>用于对集合进行映射，使用ofType确定类型
4. 在idea中添加mapper.xml文件模板
   

学习视频：
Mybatis教程IDEA版-4天-2018黑马SSM

学习资料：
Mybatis教程-实战看这一篇就够了