During project development, there are many multi-table nested queries. Today we will explain the use of multi-table queries in mybatis.
1. Preparation of database tables. This blog uses three tables (t_group (group information), t_person (person information), t_user (user information corresponding to personnel))
t_group table structure and test data 
t_person table structure and test data 
t_user table structure and test data 
2. The entity class of the corresponding table
Case 1: There is a 1:1 relationship between personnel and their corresponding users
1）Group.java
public class Group {
private int id;
private String groupName;
private String description;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getGroupName() {
return groupName;
}
public void setGroupName(String groupName) {
this.groupName = groupName;
}
public String getDescription() {
return description;
}
public void setDescription(String description) {
this.description = description;
}
}
Person.java
public class Person {
private int id;
private String name;
private String address;
private String telphone;
private String email;
private int gid;
private User user;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
public String getTelphone() {
return telphone;
}
public void setTelphone(String telphone) {
this.telphone = telphone;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public int getGid() {
return gid;
}
public void setGid(int gid) {
this.gid = gid;
}
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
}
User.java
public class User {
private int id;
private String userName;
private String password;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getUserName() {
return userName;
}
public void setUserName(String userName) {
this.userName = userName;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
}
2) Create a new sql mapping file
<?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.zlt.mybatis.model.Group">
<!--
1:1 query the user information corresponding to the person
select p.id ,p.name,p.telphone,p.address,p.email,p.gid,
u.id uid,u.user_name,u.password, u.pid
from t_person p left join t_user u on u.pid = p.id
where p.id = 1
-->
<select id="findPersonByPID" parameterType="int" resultMap="PersonAndUser">
select p.id,p.name,p.telphone,p.address,p.email,p.gid,
u.id uid,u.user_name,u.password, u.pid
from t_person p left join t_user u on u.pid = p.id
where p.id = #{id}
</select>
<!-- The first way to write -->
<resultMap type="com.zlt.mybatis.model.Person" id="PersonAndUser">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="telphone" column="telphone" />
<result property="address" column="address"/>
<result property="email" column="email"/>
<result property="gid" column="gid"/>
<association property="user" javaType="com.zlt.mybatis.model.User">
<id property="id" column="id"/>
<result property="userName" column="user_name"/>
<result property="password" column="password"/>
</association>
</resultMap>
<!-- The second way of writing -->
<!--
<resultMap type="com.zlt.mybatis.model.Person" id="person">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="telphone" column="telphone" />
<result property="address" column="address"/>
<result property="email" column="email"/>
<result property="gid" column="gid"/>
</resultMap>
<resultMap type="com.zlt.mybatis.model.Person" id="PersonAndUser" extends="person">
<association property="user" javaType="com.zlt.mybatis.model.User">
<id property="id" column="id"/>
<result property="userName" column="user_name"/>
<result property="password" column="password"/>
</association>
</resultMap>
-->
</mapper>
3) Register the sql mapping file in the mybatis configuration file
<mappers>
<mapper resource="com/zlt/mybatis/mapping/GroupPersonUserMapper.xml" />
</mappers>
4) Unit testing
public void testPersonAndUserByPID(){
SqlSession session = null;
try{
session = sqlSessionFactory.openSession();
String statement = "com.zlt.mybatis.model.Group.findPersonByPID";
Person person = session.selectOne(statement, 3);
if(null != person) {
StringBuffer sb = new StringBuffer();
sb.append("id:" + person.getId() + "\t")
.append("name:" + person.getName() +"\t")
.append("telphone:" + person.getTelphone() +"\t")
.append("email:" + person.getEmail() + "\t")
.append("gid:" + person.getGid() + "\t");
User user = person.getUser();
sb.append("id:" + user.getId() + "\t")
.append("userName:" + user.getUserName() + "\t")
.append("password:" + user.getPassword() + "\t");
System.out.println(sb.toString());
}
}finally{
if(null != session) session.close();
}
}
5) Running result
id:3 name:王二 telphone:10001 email:[email protected] gid:6 id:3 userName:wanger password:123456
Note: This is to maintain user information from the perspective of personnel, and vice versa can also maintain personnel information from the perspective of users