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
- 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
- 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>
- 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 - 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