Mybatis supports one-to-multiple database relationships by using the "set nested query" and "set nested result" methods.

Mybatis supports one-to-multiple database relationships by using the "set nested query" and "set nested result" methods.

The two entity classes are as follows: User class and Goods Commodity class. One user corresponds to multiple products (one to multiple)

package com.leo.entity;import java.util.List;public class User {private Integer id;private String username;private Integer age;private String address;private List<Goods> goodsList;public List<Goods> getGoodsList() {return goodsList;}public void setGoodsList(List<Goods> goodsList) {this.goodsList = goodsList;}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 Integer getAge() {return age;}public void setAge(Integer age) {this.age = age;}public String getAddress() {return address;}public void setAddress(String address) {this.address = address;}public User() {super();// TODO Auto-generated constructor stub}@Overridepublic String toString() {return "User [id=" + id + ", username=" + username + ", age=" + age+ ", address=" + address + ", goodsList=" + goodsList + "]";}}

Goods Product

package com.leo.entity;public class Goods {private Integer id;private String goodsName;private Integer goodsNumber;private Integer user_id;public Integer getId() {return id;}public void setId(Integer id) {this.id = id;}public String getGoodsName() {return goodsName;}public void setGoodsName(String goodsName) {this.goodsName = goodsName;}public Integer getGoodsNumber() {return goodsNumber;}public void setGoodsNumber(Integer goodsNumber) {this.goodsNumber = goodsNumber;}public Integer getUser_id() {return user_id;}public void setUser_id(Integer user_id) {this.user_id = user_id;}}

Mapper ing file of the User object class: UserDao. xml

<? Xml version = "1.0" encoding = "UTF-8"?> <! DOCTYPE mapper PUBLIC "-// mybatis.org//DTD Mapper 3.0 //" http://mybatis.org/dtd/mybatis-3-mapper.dtd "> <mapper namespace =" com. leo. mapper. userDao "> <resultMap type =" User "id =" userMap "> <id column =" u_id "property =" id "/> <result column =" username "property =" username "/> <result column =" age "property =" age "/> <result column =" address "property =" address "/> <! -- When the relationship between tables is one-to-many, use collection --> <! -- Column = "u_id" here is used to pass parameters to the nested query select = "..... "--> <collection property =" goodsList "ofType =" Goods "column =" u_id "select =" com. leo. mapper. goodsDao. selectGoodsForUser "/> </resultMap> <! -- GoodsList is a set of private properties in the User object class --> <select id = "getUserinfoById" parameterType = "int" resultMap = "userMap"> select u. id as u_id, u. username, u. age, u. address fromuser u where u. id =$ {value }; </select> </mapper>

Mapper ing file of Goods object class: GoodsDao. xml

<? Xml version = "1.0" encoding = "UTF-8"?> <! DOCTYPE mapper PUBLIC "-// mybatis.org//DTD Mapper 3.0 //" http://mybatis.org/dtd/mybatis-3-mapper.dtd "> <! -- This is the nested query ing --> <mapper namespace = "com. leo. mapper. goodsDao "> <select id =" selectGoodsForUser "parameterType =" int "resultType =" Goods "> SELECT id, goodsName, goodsNumber, user_id FROM Goods WHERE user_id =#{ value} </select> </mapper>

Environment profile mabatis-config.xml for mabatis

<? Xml version = "1.0" encoding = "UTF-8"?> <! DOCTYPE configuration PUBLIC "-// mybatis.org//DTD Config 3.0 //" http://mybatis.org/dtd/mybatis-3-config.dtd "> <configuration> <! -- I put the data source content in the db. properties file --> <properties resource = "com/leo/resources/db. properties"/> <! -- Start-type alias: mapper. the resultType in xml gets an alias, which is not very redundant --> <typeAliases> <typeAlias alias = "User" type = "com. leo. entity. user "/> <typeAlias alias =" Goods "type =" com. leo. entity. goods "/> </typeAliases> <! -- End-type alias --> <! -- Start-environments configuration --> <environments default = "development"> <environment id = "development"> <transactionManager type = "JDBC"/> <dataSource type = "POOLED"> <property name = "driver" value = "$ {driverClass}"/> <! -- Data source configuration --> <property name = "url" value = "$ {url}"/> <property name = "username" value = "$ {username}"/> <property name = "password" value = "$ {password}"/> </dataSource> </environment> </environments> <! -- End-environments configuration --> <! -- Connect to the ing file resource of the object class --> <mappers> <mapper resource = "com/leo/entity/UserDao. xml "/> <mapper resource =" com/leo/entity/GoodsDao. xml "/> </mappers> </configuration>

Tested servlet (you can also use the main function for testing)

package com.leo.servlet;import java.io.IOException;import java.io.InputStream;import java.util.List;import javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.ResultHandler;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;import com.leo.entity.Goods;import com.leo.entity.User;import com.leo.mapper.GoodsDao;import com.leo.mapper.UserDao;/** * Servlet implementation class MybatisServlet */@WebServlet("/MybatisServlet")public class MybatisServlet extends HttpServlet {private static final long serialVersionUID = 1L;protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {InputStream is = Resources.getResourceAsStream("com/leo/resources/mybatis-config.xml");SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(is);SqlSession session = factory.openSession();//UserDao ud = session.getMapper(UserDao.class);GoodsDao gd = session.getMapper(GoodsDao.class);List<Goods> goodsList= gd.selectGoodsForUser(1);//User user = ud.getUserinfoById(1);//System.out.println(user);//List<Goods> goodsList  =  user.getGoodsList();for (Goods goods : goodsList) {System.out.println(goods.getId()+"   "+ goods.getGoodsName()+"   "+goods.getGoodsNumber()+ "  "+ goods.getUser_id());}session.commit();session.close();}protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {doGet(request, response);}}

The above is a set nested query, and another method is the set nested results. This method only requires one object class file, which is a cascade query that is automatically completed.

The following method uses the nested result of the Set:

You only need to modify UserDao. xml and use this ing file.

<? Xml version = "1.0" encoding = "UTF-8"?> <! DOCTYPE mapper PUBLIC "-// mybatis.org//DTD Mapper 3.0 //" http://mybatis.org/dtd/mybatis-3-mapper.dtd "> <mapper namespace =" com. leo. mapper. userDao "> <resultMap type =" Goods "id =" goodsMap "> <id column =" g_id "property =" id "/> <result column =" goodsName "property =" goodsName "/> <result column =" goodsNumber "property =" goodsNumber "/> <result column =" user_id "property =" user_id "/> </resultMap> <resultMap type = "User" Id = "userMap"> <id column = "u_id" property = "id"/> <result column = "username" property = "username"/> <result column = "age "property =" age "/> <result column =" address "property =" address "/> <collection property =" goodsList "ofType =" Goods "resultMap =" goodsMap "/> <! -- The difference between the two methods is here, you can find out by yourself --> </resultMap> <select id = "getUserinfoById" parameterType = "int" resultMap = "userMap"> select u. id as u_id, u. username, u. age, u. address, g. id as g_id, <! -- Nested results This method uses a join query, while nested query uses two --> g. goodsName, g. goodsNumber, g. user_id fromuser uinner join goods g on u. id = g. user_id where u. id =$ {value }; </select> </mapper>

I hope this will help you. If you have any incorrect wording, I hope you can correct it and make progress.