Mybatis [14] - how does mybatis implement one to many query?

Letter 16 2021-01-23 15:44:00
mybatis mybatis implement query

++ notes : Code is hosted on GitHub On , The address is : , The project is mybatis-10-one2many, You need to take it yourself , Need configuration maven The environment and mysql Environmental Science (sql Statements in resource Under the test.sql in ), If you think it's useful, you can order a little star .

docsify The document address is :

A lot of times , When the query condition involves multiple tables with association relationship , You need to use association queries , There are four kinds of association queries .

  • One to one association query
  • One to many associated queries
  • Many to one association query
  • Many to many associated query

Next, we need to practice one to many association query , The so-called one to many is that the attributes in an object are a collection of objects . For example, each country has several leaders . The corresponding country table and the leader's table .

Create data table

# Create database
# Create data table
CREATE TABLE `test`.`minister` ( `mid` INT(10) NOT NULL AUTO_INCREMENT ,`mname` VARCHAR(20) NOT NULL ,`countryId` INT(20) NOT NULL ,PRIMARY KEY(`mid`)) ENGINE = MyISAM;
# Initialize data table
INSERT INTO `country` (`cid`, `cname`) VALUES ('1', 'USA');
INSERT INTO `country` (`cid`, `cname`) VALUES ('2', 'England');
INSERT INTO `minister` (`mid`, `mname`, `countryId`) VALUES ('1', 'aaa', '1');
INSERT INTO `minister` (`mid`, `mname`, `countryId`) VALUES ('2', 'bbb', '1');
INSERT INTO `minister` (`mid`, `mname`, `countryId`) VALUES ('3', 'ccc', '2');
INSERT INTO `minister` (`mid`, `mname`, `countryId`) VALUES ('4', 'ddd', '2');
INSERT INTO `minister` (`mid`, `mname`, `countryId`) VALUES ('5', 'eee', '2');

Entity class

country The corresponding entity class , Attributes are :cid,cname, as well as miniters, Realization get() as well as set() Method ,tostring() Method :

import java.util.Set;
public class Country {
private Integer cid;
private String cname;
private Set<Minister> ministers;
public Integer getCid() {
return cid;
public void setCid(Integer cid) {
this.cid = cid;
public String getName() {
return cname;
public void setName(String cname) {
this.cname = cname;
public Set<Minister> getMinisters() {
return ministers;
public void setMinisters(Set<Minister> ministers) {
this.ministers = ministers;
public String toString() {
return "Country [cid=" + cid + ", cname=" + cname + ", ministers="
+ ministers + "]";

Minister The entity class , It also needs to be realized set() as well as get() Method ,toString() Method :

public class Minister {
private Integer mid;
private String mname;
public String toString() {
return "Minister [mid=" + mid + ", mname=" + mname + "]";
public Integer getMid() {
return mid;
public void setMid(Integer mid) {
this.mid = mid;
public String getMname() {
return mname;
public void setMname(String mname) {
this.mname = mname;

stay mybatis.xml It's registered in the file mapper file :

 <!-- Register map file -->
<mapper resource="mapper/mapper.xml"/>

We operate the database interface :

public interface ICountryDao {
Country selectCountryById(int cid);
Country selectCountryById2(int cid);

mapper.xml file , There are two corresponding ways to realize one to many query :

  • One is nested queries ( Query multiple tables separately ), That is, there is an entrance select sentence , But this sentence only selects country Information about , stay resultMap It's customized , Including a <collection></collection>, Specify the corresponding type in it ,property It's the name of the attribute ,ofType It's the type of this genus ,select Specify another... To call select sentence ,column It's the input parameter , This way you can achieve delayed loading , That's when it's not available , It won't call the SQL. In this case, multiple tables can be queried individually across multiple mapper file , Just write the corresponding namespace That's all right.
  • Results nested query ( Multi table join query ): There is also an entrance select sentence , What is different from the above is , This select Statement selects the fields of both tables , We need to specify one resultMap, Just like other sub tags , however <collection></collection> It doesn't call the other sql sentence , Just specify the corresponding attribute and field .
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
PUBLIC "-// Mapper 3.0//EN"
<mapper namespace="dao.ICountryDao">
<!-- resultMap It can solve the problem of different fields and properties -->
<!-- It will be used more in the future , Because you can use deferred loading -->
<!-- nested queries -->
<select id="selectMinisterByCountry" resultType="Minister">
select mid,mname from minister where countryId=#{ooo}
<resultMap type="Country" id="countryMapper">
<id column="cid" property="cid"/>
<result column="cname" property="cname"/>
<!-- country One of the member variables in is ministers, Its generics are Minister -->
<collection property="ministers"
<select id="selectCountryById" resultMap="countryMapper">
select cid,cname
from country
<!-- Nesting results -->
<select id="selectCountryById2" resultMap="countryMapper2">
select * from country c,minister m where c.cid = m.countryId and c.cid= #{cid}
<resultMap id="countryMapper2" type="Country">
<id column="cid" property="cid"/>
<result column="cname" property="cname"/>
<collection property="ministers" ofType="Minister">
<id property="mid" column="mid"/>
<result property="mname" column="mname"/>

test test:

public class MyTest {
private ICountryDao dao;
private SqlSession sqlSession;
public void Before(){
public void TestselectCountryById(){
Country country=dao.selectCountryById(1);
public void TestselectCountryById2(){
Country country=dao.selectCountryById2(1);
public void after(){

Tool classes used

public class MyBatisUtils {
static private SqlSessionFactory sqlSessionFactory;
static public SqlSession getSqlSession() {
InputStream is;
try {
is = Resources.getResourceAsStream("mybatis.xml");
if (sqlSessionFactory == null) {
sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);
return sqlSessionFactory.openSession();
} catch (IOException e) {
// TODO Auto-generated catch block
return null;

The results of the two interfaces are consistent :

[service] 2018-07-12 14:01:15,835 - org.apache.ibatis.transaction.jdbc.JdbcTransaction -508 [main] DEBUG org.apache.ibatis.transaction.jdbc.JdbcTransaction - Setting autocommit to false on JDBC Connection [com.mysql.jdbc.JDBC4Connection@4abdb505]
[service] 2018-07-12 14:01:15,837 - dao.ICountryDao.selectCountryById2 -510 [main] DEBUG dao.ICountryDao.selectCountryById2 - ==> Preparing: select * from country c,minister m where c.cid = m.countryId and c.cid= ?
[service] 2018-07-12 14:01:15,869 - dao.ICountryDao.selectCountryById2 -542 [main] DEBUG dao.ICountryDao.selectCountryById2 - ==> Parameters: 1(Integer)
[service] 2018-07-12 14:01:15,884 - dao.ICountryDao.selectCountryById2 -557 [main] DEBUG dao.ICountryDao.selectCountryById2 - <== Total: 2
Country [cid=1, cname=USA, ministers=[Minister [mid=2, mname=bbb], Minister [mid=1, mname=aaa]]]

【 Author's brief introduction 】
Qin Huai , official account 【 Qin Huai grocery store 】 author , The road to technology is not in the moment , one's nobility lasts forever , Even slowly , Go on and on . The world wants everything to be quick , faster , But I hope I can take every step well , Write every article well , Looking forward to communicating with you .

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