1. MyBatis关联查询简介

　　MyBatis中级联分为3中：association、collection及discriminator。

　　◊ association：一对一关联

　　◊ collection：一对多关联

　　◊ discriminator：鉴别器，可以根据实际选择采用哪个类作为实例，允许根据特定的条件去关联不同的结果集。

2. 一对一关联查询

　　表结构设计：user、user_profile

2.1 方式一

<?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.libing.helloworld.dao.IUserProfileDao"><resultMap id="baseResultMap" type="com.libing.helloworld.model.UserProfile"><id property="id" column="profile_id" /><result property="userId" column="user_id" /><result property="name" column="name" /><result property="phone" column="phone" /><result property="email" column="email" /><result property="address" column="address" /><association property="user" javaType="com.libing.helloworld.model.User"><id property="id" column="id"/><result property="userName" column="user_name" /><result property="password" column="password" /></association></resultMap><select id="findById" resultMap="baseResultMap">SELECTu.id,u.user_name,u.`password`,user_profile.id profile_id,user_profile.user_id,user_profile.`name`,user_profile.phone,user_profile.email,user_profile.addressFROMuser_profile,`user` uWHEREuser_profile.user_id = u.idAND user_profile.id = #{id}</select>
</mapper>

DEBUG [main] - ==>  Preparing: SELECT u.id, u.user_name, u.`password`, user_profile.id profile_id, user_profile.user_id, user_profile.`name`, user_profile.phone, user_profile.email, user_profile.address FROM user_profile, `user` u WHERE user_profile.user_id = u.id AND user_profile.id = ?
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <==      Total: 1

2.2 方式二

　　UserMapper.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.libing.helloworld.dao.IUserDao"><resultMap id="baseResultMap" type="com.libing.helloworld.model.User"><id property="id" column="id" /><result property="userName" column="user_name" /><result property="password" column="password" /></resultMap><select id="findById" resultMap="baseResultMap">SELECTid,user_name,passwordFROMuserWHERE id = #{id}</select>
</mapper>

　　UserProfileMapper.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.libing.helloworld.dao.IUserProfileDao"><resultMap id="baseResultMap" type="com.libing.helloworld.model.UserProfile"><id property="id" column="id" /><result property="userId" column="user_id" /><result property="name" column="name" /><result property="phone" column="phone" /><result property="email" column="email" /><result property="address" column="address" /><association property="user" column="user_id" select="com.libing.helloworld.dao.IUserDao.findById" /></resultMap><select id="findById" resultMap="baseResultMap">SELECTid,user_id,name,phone,email,addressFROMuser_profileWHERE id = #{id}</select>
</mapper>

　　UserProfileTest.java：

package com.libing.helloworld.test;import java.io.InputStream;import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.apache.log4j.PropertyConfigurator;
import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;import com.libing.helloworld.dao.IUserProfileDao;
import com.libing.helloworld.model.UserProfile;public class UserProfileTest {SqlSession sqlSession = null;@Beforepublic void init() {PropertyConfigurator.configure(UserProfileTest.class.getClassLoader().getResourceAsStream("log4j.properties"));String resource = "mybatis-config.xml";InputStream inputStream = UserProfileTest.class.getClassLoader().getResourceAsStream(resource);SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);sqlSession = sqlSessionFactory.openSession();}@Testpublic void findById() {try {IUserProfileDao userProfileDao = sqlSession.getMapper(IUserProfileDao.class);UserProfile userProfile = userProfileDao.findById(1);Assert.assertNotNull(userProfile);} catch (Exception e) {e.printStackTrace();} finally {sqlSession.close();}}}

　　运行执行的SQL语句：

DEBUG [main] - ==>  Preparing: SELECT id, user_id, name, phone, email, address FROM user_profile WHERE id = ?
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - ====>  Preparing: SELECT id, user_name, password FROM user WHERE id = ?
DEBUG [main] - ====> Parameters: 1(Integer)
DEBUG [main] - <====      Total: 1
DEBUG [main] - <==      Total: 1

3. 一对多关联查询

　　表结构设计：user、task

3.1 方式一

3.2 方式二

package com.libing.helloworld.model;import java.util.List;public class User {private int id;private String userName;private String password;private List<Task> tasks;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;}public List<Task> getTasks() {return tasks;}public void setTasks(List<Task> tasks) {this.tasks = tasks;}}

User.java

package com.libing.helloworld.model;public class Task {private int id;private int userId;private String taskName;private String content;public int getId() {return id;}public void setId(int id) {this.id = id;}public int getUserId() {return userId;}public void setUserId(int userId) {this.userId = userId;}public String getTaskName() {return taskName;}public void setTaskName(String taskName) {this.taskName = taskName;}public String getContent() {return content;}public void setContent(String content) {this.content = content;}}

Task.java

<?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.libing.helloworld.dao.ITaskDao"><resultMap id="baseResultMap" type="com.libing.helloworld.model.Task"><id property="id" column="id" /><result property="userId" column="user_id" /><result property="taskName" column="task_name" /><result property="content" column="content" /></resultMap><select id="findTasksByUserId" resultMap="baseResultMap">SELECTid,user_id,task_name,contentFROMtaskWHERE user_id = #{userId}</select>
</mapper>

TaskMapper.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.libing.helloworld.dao.IUserDao"><resultMap id="baseResultMap" type="com.libing.helloworld.model.User"><id property="id" column="id" /><result property="userName" column="user_name" /><result property="password" column="password" /><collection property="tasks" column="id" select="com.libing.helloworld.dao.ITaskDao.findTasksByUserId"></collection></resultMap><select id="findById" resultMap="baseResultMap">SELECTid,user_name,passwordFROMuserWHERE id = #{id}</select>
</mapper>

@Test
public void findAll() {try {IUserDao userDao = sqlSession.getMapper(IUserDao.class);User user = userDao.findById(1);Assert.assertNotNull(user);} catch (Exception e) {e.printStackTrace();} finally {sqlSession.close();}
}

　　执行的SQL语句：

DEBUG [main] - ==>  Preparing: SELECT id, user_name, password FROM user WHERE id = ?
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - ====>  Preparing: SELECT id, user_id, task_name, content FROM task WHERE user_id = ?
DEBUG [main] - ====> Parameters: 1(Integer)
DEBUG [main] - <====      Total: 0
DEBUG [main] - <==      Total: 1

4. N + 1问题

5. 延迟加载

转载于:https://www.cnblogs.com/libingql/p/7517310.html

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