本文只用於個人複習mybatis使用:
mybatis中的多表查詢,一對一,一對多,多對多
數據庫表如下:
user表
account表
role表
1 、編寫實體
創建實體:包含一對多,和多對多的實體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;
/**
* 一對多關係映射:主表實體應該包含從表實體的集合引用
*/
private List<Account> accounts;
/**
* 多對多關係映射:兩個實體類都必須包含對方的一個集合引用
* (多對多關係其實我們看成是雙向的一對多關係)
*/
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 +
'}';
}
}
實體: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 +
'}';
}
}
多對多實體: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;
/**
* 多對多關係映射:兩個實體類都必須包含對方的一個集合引用
* (多對多關係其實我們看成是雙向的一對多關係)
*/
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 、編寫映射
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">
<!-- 定義可以封裝帶有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>
<!-- 一對一的關係映射,配置封裝user的內容 -->
<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>
<!-- 查詢所有 -->
<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">
<!-- 配置從用戶到角色的多對多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對象中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>
<!-- 查詢所有用戶 -->
<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、運行結果
多對多
4、總結
- 處理多對多關係時,需要避免在resultMap中出現兩個類出現相同的主鍵,否則會導致相同主鍵的多對多關係只顯示一條數據
- mybatis_config.xml中可以使用<package>標籤以包的方式來添加映射
<!-- 指定映射配置文件的位置 -->
<mappers>
<package name="com.cimo.DAO"/>
</mappers>
- resultMap中<association>和<collection>的區別
<association>用於對單獨的類進行映射,使用javaType確定類型
<collection>用於對集合進行映射,使用ofType確定類型 - 在idea中添加mapper.xml文件模板
學習視頻:
Mybatis教程IDEA版-4天-2018黑馬SSM
學習資料:
Mybatis教程-實戰看這一篇就夠了