Mybatis学习笔记（二）

八、多表联合查询

(一) 多表联合查询概述

在开发过程中单表查询不能满足项目需求分析功能，对于复杂业务来讲，关联的表有几张，甚至几十张并且表与表之间的关系相当复杂。为了能够实业复杂功能业务，就必须进行多表查询，在mybatis中提供了多表查询的结果时映射标签，可以实现表之间的一对一、一对多、多对一、多对多关系映射。

(二) MyBatis实现一对一查询

1. 构建数据库表

        person(个人表) IdCard(身份证表)

CREATE TABLE person(
p_id INT NOT NULL AUTO_INCREMENT,
p_name VARCHAR(30),
PRIMARY KEY(p_id) 
);#IdCard表
CREATE TABLE idcard(
c_id INT NOT NULL AUTO_INCREMENT,
c_cardno VARCHAR(18),
c_uselife DATE,
c_person_id INT NOT NULL,
PRIMARY KEY(c_id),
FOREIGN KEY(c_person_id) REFERENCES person(p_id),
UNIQUE KEY(c_cardno));INSERT INTO person(p_name) VALUES('张三'),('李四');INSERT INTO idcard(c_cardno,c_uselife,c_person_id)
VALUES('110112199012127821','2029-10-10',1);
INSERT INTO idcard(c_cardno,c_uselife,c_person_id)
VALUES('120114199911103491','2030-12-01',2);

2.准备项目环境

3.嵌套结果方式查询

 3.1实体类创建

Person

package com.jn.entity;public class Person {private Integer id;private String name;public Person() {}public Integer getId() {return id;}public void setId(Integer id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}@Overridepublic String toString() {return "Person{" +"id=" + id +", name='" + name + '\'' +'}';}
}

IdCard 

package com.jn.entity;import java.util.Date;public class IdCard {private Integer id;private String cardno;private Date useLife;public IdCard() {}public Integer getId() {return id;}public void setId(Integer id) {this.id = id;}public String getCardno() {return cardno;}public void setCardno(String cardno) {this.cardno = cardno;}public Date getUserLife() {return useLife;}public void setUserLife(Date userLife) {this.useLife = userLife;}@Overridepublic String toString() {return "IdCard{" +"id=" + id +", cardno='" + cardno + '\'' +", userLife=" + useLife +'}';}
}

3.2编写sql语句

实现查询个人信息时，也要查询个人所对应的身份证信息。

select p.*,c.* from
person p,
idcard c
where p.p_id=c.c_person_id and p.p_id=1;

3.3编写PersonIdCard类

package com.jn.entity;import java.util.Date;public class PersonIdCard extends Person{private String cardno;private Date useLife;public String getCardno() {return cardno;}public void setCardno(String cardno) {this.cardno = cardno;}public Date getUseLife() {return useLife;}public void setUseLife(Date useLife) {this.useLife = useLife;}
}

3.4定义持久层接口

PersonDao

package com.jn.dao;import com.jn.entity.PersonIdCard;public interface PersonDao {public PersonIdCard getPersonById(int id);
}

3.5定义 PersonDao.xml 

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.jn.dao.PersonDao"><select id="getPersonById" resultMap="PersonResultMap">SELECT p.*,c.* from person p ,idcard c where p.p_id = c.c_person_id and p.p_id = 1;</select><resultMap id="PersonResultMap" type="PersonIdCard"><id column="p_id" property="id"></id><result column="p_id" property="id"></result><result column="p_name" property="name"></result><result column="c_cardno" property="cardno"></result><result column="c_uselife" property="useLife"></result></resultMap>
</mapper>

3.6创建 PersonTest 测试类

package com.jn.test;import com.jn.dao.PersonDao;
import com.jn.entity.PersonIdCard;
import com.jn.utils.MyBatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;public class PersonTest {@Testpublic void testGetPersonById() throws Exception{//获取sqlSession对象SqlSession sqlSession = MyBatisUtils.getSession();//通过sqlSession获取PersonDao的代理对象PersonDao personDao = sqlSession.getMapper(PersonDao.class);PersonIdCard personIdCard = personDao.getPersonById(1);System.out.println(personIdCard);//commitsqlSession.close();MyBatisUtils.close(sqlSession);}
}

3.7测试结果

4.嵌套查询方式

前言

查的到底是什么？

        查的是一个对象Person，这个对象不仅包含了Person里面的属性，同时还包含了一个IdCard对象。然后把两个表里面的信息整合以便于后续处理。

4.1添加idCard属性

在person类里面添加idCard属性 

package com.jn.entity;public class Person {private Integer id;private String name;private IdCard idCard;public Person() {}public Integer getId() {return id;}public void setId(Integer id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public IdCard getIdCard() {return idCard;}public void setIdCard(IdCard idCard) {this.idCard = idCard;}@Overridepublic String toString() {return "Person{" +"id=" + id +", name='" + name + '\'' +", idCard=" + idCard +'}';}
}

4.2持久层里面添加方法

package com.jn.dao;import com.jn.entity.Person;
import com.jn.entity.PersonIdCard;public interface PersonDao {public PersonIdCard getPersonById(Integer id);//嵌套查询的查询方法public Person getPersonById2(Integer id);
}

4.3持久层接口IdCardDao

package com.jn.dao;import com.jn.entity.Person;public interface IdCardDao {public Person getIdCardByPersonId(Integer id);
}

4.4定义 IdCardDao.xml 

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.jn.dao.IdCardDao"><!--查询符合结果的IdCard对象并返回给命名空间IdCardDao下的getIdCardByPersonId函数，然后配合PersonDao.xml进行数据的联合查询
--><select id="getIdCardByPersonId"  parameterType="int" resultMap="IdCardResult">select * from idcard  where c_person_id  = #{id}</select><resultMap id="IdCardResult" type="IdCard"><id column="c_id" property="id"></id><result column="c_id" property="id"></result><result column="c_cardno" property="cardno"></result><result column="c_uselife" property="useLife"></result></resultMap>
</mapper>

4.5PersonDao.xml 配置

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.jn.dao.PersonDao"><!--嵌套结果的查询--><select id="getPersonById" resultMap="PersonResultMap">SELECT p.*,c.* from person p ,idcard c where p.p_id = c.c_person_id and p.p_id = 1;</select><resultMap id="PersonResultMap" type="PersonIdCard"><id column="p_id" property="id"></id><result column="p_id" property="id"></result><result column="p_name" property="name"></result><result column="c_cardno" property="cardno"></result><result column="c_uselife" property="useLife"></result></resultMap><!--嵌套查询的结果。根据id先查询到Person对象里面的字段信息，然后配合IdCard.xml查询返回的IdCard对象进行结合，然后返回一个getPersonById2函数的Person对象--><select id="getPersonById2" parameterType="int" resultMap="PersonResultMap2">select * from person where p_id = #{id}</select><resultMap id="PersonResultMap2" type="Person"><id column="p_id" property="id"></id><result column="p_name" property="name"></result><!--映射Person的复杂字段idCard对象属性--><association property="idCard" javaType="IdCard" column="p_id" select="com.jn.dao.IdCardDao.getIdCardByPersonId"></association></resultMap>
</mapper>

column:表示取上次查询出来的指定列的值，做为select属性所指定的查询的输入值。
select:表示指定的查询.

4.6加入测试方法

    //测试嵌套查询@Testpublic void testNestedQueryById() throws Exception{//获取sqlSession对象SqlSession sqlSession = MyBatisUtils.getSession();//通过sqlSession获取PersonDao的代理对象PersonDao personDao = sqlSession.getMapper(PersonDao.class);Person person = personDao.getPersonById2(1);System.out.println(person);MyBatisUtils.close(sqlSession);}

4.7测试结果

 到目前为止项目的结构目录

(三)MyBatis实现一对多查询

1.创建数据库表

department(部门表),employee(员工表)同时设定部门和员工表的关系

CREATE TABLE department(
d_id INT NOT NULL AUTO_INCREMENT,
d_name VARCHAR(100),
PRIMARY KEY(d_id)
);CREATE TABLE employee(
e_id INT NOT NULL AUTO_INCREMENT,
e_name VARCHAR(30),
e_gender VARCHAR(6),
e_age INT,
e_depart_id INT,
PRIMARY KEY(e_id),
FOREIGN KEY(e_depart_id) REFERENCES department(d_id)
);
-- 向 department 表中插入数据
INSERT INTO department (d_name) VALUES ('研发部');
INSERT INTO department (d_name) VALUES ('销售部');
INSERT INTO department (d_name) VALUES ('财务部');
INSERT INTO department (d_name) VALUES ('市场部');
INSERT INTO department (d_name) VALUES ('人力资源部');-- 向 employee 表中插入数据
INSERT INTO employee (e_name, e_gender, e_age, e_depart_id) VALUES ('张三', '男', 25, 1);
INSERT INTO employee (e_name, e_gender, e_age, e_depart_id) VALUES ('李四', '女', 30, 1);
INSERT INTO employee (e_name, e_gender, e_age, e_depart_id) VALUES ('王五', '男', 28, 2);
INSERT INTO employee (e_name, e_gender, e_age, e_depart_id) VALUES ('赵六', '女', 32, 2);
INSERT INTO employee (e_name, e_gender, e_age, e_depart_id) VALUES ('孙七', '男', 27, 3);
INSERT INTO employee (e_name, e_gender, e_age, e_depart_id) VALUES ('周八', '男', 26, 1);
INSERT INTO employee (e_name, e_gender, e_age, e_depart_id) VALUES ('吴九', '女', 29, 1);
INSERT INTO employee (e_name, e_gender, e_age, e_depart_id) VALUES ('郑十', '男', 31, 2);
INSERT INTO employee (e_name, e_gender, e_age, e_depart_id) VALUES ('钱十一', '女', 24, 3);
INSERT INTO employee (e_name, e_gender, e_age, e_depart_id) VALUES ('刘十二', '男', 33, 4);
INSERT INTO employee (e_name, e_gender, e_age, e_depart_id) VALUES ('陈十三', '女', 28, 4);
INSERT INTO employee (e_name, e_gender, e_age, e_depart_id) VALUES ('杨十四', '男', 30, 5);
INSERT INTO employee (e_name, e_gender, e_age, e_depart_id) VALUES ('胡十五', '女', 27, 5);​

2.嵌套结果的方式

2.1实体类创建

Department类

package com.jn.entity;public class Department {private int id;private String name;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;}
}

Employee类

package com.jn.entity;public class Employee {private int id;private String name;private String gender;private Integer age;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 getGender() {return gender;}public void setGender(String gender) {this.gender = gender;}public Integer getAge() {return age;}public void setAge(Integer age) {this.age = age;}@Overridepublic String toString() {return "Employee{" +"id=" + id +", name='" + name + '\'' +", gender='" + gender + '\'' +", age=" + age +'}';}
}

2.2编写sql查询语句

select d.*,e.* from department d,employee  e where d.d_id=e.e_depart_id and d.d_id=1;

2.3Department加入List

把Employee属性变为List集合作为Department的属性

package com.jn.entity;import java.util.List;public class Department {private int id;private String name;private List<Employee> emps;public Department() {}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 List<Employee> getEmps() {return emps;}public void setEmps(List<Employee> emps) {this.emps = emps;}@Overridepublic String toString() {return "Department{" +"id=" + id +", name='" + name + '\'' +", emps=" + emps +'}';}
}

2.4持久层DepartmentDao 

package com.jn.dao;import com.jn.entity.Department;public interface DepartmentDao {public Department getDepartById(Integer id);
}

2.5映射文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.jn.dao.DepartmentDao"><select id="getDepartById" parameterType="int" resultMap="getDepartmentMap">select d.*,e.* from department d,employee  e where d.d_id=e.e_depart_id and d.d_id=#{id}</select><resultMap id="getDepartmentMap" type="Department"><id column="d_id" property="id"></id><result column="d_name" property="name"></result><collection property="emps" ofType="Employee"><id column="e_id" property="id"></id><result column="e_name" property="name"></result><result column="e_gender" property="gender"></result><result column="e_age" property="age"></result></collection></resultMap>
</mapper>

2.6测试方法

package com.jn.test;import com.jn.dao.DepartmentDao;
import com.jn.entity.Department;
import com.jn.utils.MyBatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;public class DepartmentTest {@Testpublic void testGetDepartmentById() throws Exception{//获取sqlSession对象SqlSession sqlSession = MyBatisUtils.getSession();//通过sqlSession对象得到DepartmentDao的接口代理对象DepartmentDao departmentDao = sqlSession.getMapper(DepartmentDao.class);Department department = departmentDao.getDepartById(1);System.out.println(department.getName());department.getEmps().forEach(System.out::println);//closesqlSession.close();}
}

2.7测试结果

 

3.嵌套查询的方式

3.1定义EmployeeDao

package com.jn.dao;import com.jn.entity.Employee;import java.util.List;public interface EmployeeDao {public List<Employee> getEmployeeById(Integer id);
}

3.2定义 EmployeeDao.xml 

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.jn.dao.EmployeeDao"><!--配置employee的查询语句，返回一个Employee对象的集合--><select id="getEmployeeById" parameterType="int" resultMap="getEmployeeResultMap">select * from employee where e_depart_id = #{id}</select><resultMap id="getEmployeeResultMap" type="Employee"><id column="e_id" property="id"></id><result column="e_name" property="name"></result><result column="e_gender" property="gender"></result><result column="e_age" property="age"></result></resultMap>
</mapper>

3.3 DepartmentDao添加

package com.jn.dao;import com.jn.entity.Department;public interface DepartmentDao {public Department getDepartById(Integer id);public Department getDepartById2(Integer id);
}

3.4DepartmentDao.xml添加

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.jn.dao.DepartmentDao"><select id="getDepartById" parameterType="int" resultMap="getDepartmentMap">select d.*,e.* from department d,employee  e where d.d_id=e.e_depart_id and d.d_id=#{id}</select><resultMap id="getDepartmentMap" type="Department"><id column="d_id" property="id"></id><result column="d_name" property="name"></result><collection property="emps" ofType="Employee"><id column="e_id" property="id"></id><result column="e_name" property="name"></result><result column="e_gender" property="gender"></result><result column="e_age" property="age"></result></collection></resultMap><select id="getDepartById2" parameterType="int" resultMap="getDepartmentMap2">select * from department where d_id=#{id}</select><resultMap id="getDepartmentMap2" type="Department"><id column="d_id" property="id"></id><result column="d_name" property="name"></result><collection property="emps" ofType="Employee" column="d_id" select="com.jn.dao.EmployeeDao.getEmployeeById"></collection></resultMap>
</mapper>

3.5测试方法

    //嵌套查询@Testpublic void testGetDepartmentById2() throws Exception{//获取sqlSession对象SqlSession sqlSession = MyBatisUtils.getSession();//通过sqlSession对象得到DepartmentDao的接口代理对象DepartmentDao departmentDao = sqlSession.getMapper(DepartmentDao.class);Department department = departmentDao.getDepartById2(1);System.out.println(department.getName());department.getEmps().forEach(System.out::println);//closesqlSession.close();}

3.6测试结果

(三)MyBatis实现多对多查询

1.创建数据库表

CREATE TABLE student(sid INT NOT NULL AUTO_INCREMENT,sname VARCHAR(30),PRIMARY KEY (sid)
);CREATE TABLE teacher(tid INT NOT NULL AUTO_INCREMENT,tname VARCHAR(30),PRIMARY KEY (tid)
);CREATE TABLE student_teacher(s_id INT NOT NULL,t_id INT NOT NULL,PRIMARY KEY (s_id,t_id),FOREIGN KEY (s_id) REFERENCES student(sid),FOREIGN KEY (t_id) REFERENCES teacher(tid)
);
INSERT INTO student(sname) VALUES('张三'),('李四');
INSERT INTO teacher (tname) VALUES('刘老师'),('李老师');
INSERT INTO student(sname) VALUES('王五'),('赵六');
INSERT INTO teacher(tname) VALUES('张老师'),('王老师');
INSERT INTO student(sname) VALUES('孙七'),('周八');
INSERT INTO teacher(tname) VALUES('陈老师'),('杨老师');
INSERT INTO student(sname) VALUES('吴九'),('郑十');
INSERT INTO teacher(tname) VALUES('马老师'),('胡老师');
INSERT INTO student_teacher(s_id,t_id) VALUES(1,1),(1,2),(2,1),(3,3),(3,4),(4,3),(4,4),(5,5),(5,6),(6,5),(6,6),(7,7),(7,8),(8,7),(8,8);

2.嵌套结果方式

2.1创建数据模型

Student,Teacher,StudentTeacher

package com.jn.entity;public class Student {private int id;private int name;public int getId() {return id;}public void setId(int id) {this.id = id;}public int getName() {return name;}public void setName(int name) {this.name = name;}
}

package com.jn.entity;public class Teacher {private int id;private String name;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;}@Overridepublic String toString() {return "Teacher{" +"id=" + id +", name='" + name + '\'' +'}';}
}

package com.jn.entity;public class StudentTeacher {private int sid;private int tid;public int getSid() {return sid;}public void setSid(int sid) {this.sid = sid;}public int getTid() {return tid;}public void setTid(int tid) {this.tid = tid;}
}

2.2编写多对多的sql语句

select s.*,t.*,st.* from student s,teacher t,student_teacher st where s.sid = st.s_id and st.t_id=t.tid AND s.sid=1;

2.3S中加入List属性

package com.jn.entity;import java.util.List;public class Student {private int id;private String name;private List<StudentTeacher> studentTeacherList;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 List<StudentTeacher> getStudentTeacherList() {return studentTeacherList;}public void setStudentTeacherList(List<StudentTeacher> studentTeacherList) {this.studentTeacherList = studentTeacherList;}@Overridepublic String toString() {return "Student{" +"id=" + id +", name=" + name +", studentTeacherList=" + studentTeacherList +'}';}
}

2.4ST加入Teacher属性

package com.jn.entity;public class StudentTeacher {private int sid;private int tid;private Teacher teacher;public int getSid() {return sid;}public void setSid(int sid) {this.sid = sid;}public int getTid() {return tid;}public void setTid(int tid) {this.tid = tid;}public Teacher getTeacher() {return teacher;}public void setTeacher(Teacher teacher) {this.teacher = teacher;}@Overridepublic String toString() {return "StudentTeacher{" +"sid=" + sid +", tid=" + tid +", teacher=" + teacher +'}';}
}

2.5tudentDao编写

package com.jn.dao;import com.jn.entity.Student;public interface StudentDao  {public Student getStudentById(Integer id);}

2.6SudentDao.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.jn.dao.StudentDao"><select id="getStudentById" parameterType="int" resultMap="getStudentMap">select s.*,t.*,st.* from student s,teacher t,student_teacher st where s.sid = st.s_id and st.t_id=t.tid AND s.sid=#{id}</select><resultMap id="getStudentMap" type="Student"><id column="sid" property="id"></id><result column="sname" property="name"></result><collection property="studentTeacherList" ofType="StudentTeacher"><result column="s_id" property="sid"></result><result column="t_id" property="tid"></result><association property="teacher" javaType="Teacher"><id column="tid" property="id"></id><result column="tname" property="name"></result></association></collection></resultMap>
</mapper>

2.7测试方法

package com.jn.test;import com.jn.dao.StudentDao;
import com.jn.entity.Student;
import com.jn.entity.StudentTeacher;
import com.jn.utils.MyBatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;import java.util.List;public class StudentTest {@Testpublic void testGetStudentById()throws Exception{SqlSession sqlSession = MyBatisUtils.getSession();StudentDao studentDao = sqlSession.getMapper(StudentDao.class);Student student = studentDao.getStudentById(1);System.out.println(student.getName());List<StudentTeacher> studentTeacherList = student.getStudentTeacherList();studentTeacherList.forEach(System.out::println);}
}

2.8测试结果

3.嵌套查询方式

3.1TeacherDao

package com.jn.dao;import com.jn.entity.Teacher;public interface TeacherDao {public Teacher getTeacherById(Integer id);
}

 3.2TeacherDao.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.jn.dao.TeacherDao"><select id="getTeacherById" parameterType="int" resultType="Teacher">select tid id,tname name from teacher where tid =#{id}</select>
</mapper>

3.3StudentTeacherDao

package com.jn.dao;import com.jn.entity.StudentTeacher;import java.util.List;public interface StudentTeacherDao {public List<StudentTeacher> getStudentTeacherBySid(Integer id);
}

3.4StudentTeacherDao.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.jn.dao.StudentTeacherDao"><select id="getStudentTeacherBySid" parameterType="int" resultMap="getStudentTeacherMap">select * from student_teacher where s_id =#{id}</select><resultMap id="getStudentTeacherMap" type="StudentTeacher"><result column="s_id" property="sid"></result><result column="t_id" property="tid"></result><association property="teacher" column="t_id"  javaType="Teacher"select="com.jn.dao.TeacherDao.getTeacherById"></association></resultMap>
</mapper>

3.5StudentDao添加方法

package com.jn.dao;import com.jn.entity.Student;public interface StudentDao  {public Student getStudentById(Integer id);public Student getStudentById2(Integer id);}

3.6StudentDao.xml配置

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.jn.dao.StudentDao"><select id="getStudentById" parameterType="int" resultMap="getStudentMap">select s.*,t.*,st.* from student s,teacher t,student_teacher st where s.sid = st.s_id and st.t_id=t.tid AND s.sid=#{id}</select><resultMap id="getStudentMap" type="Student"><id column="sid" property="id"></id><result column="sname" property="name"></result><collection property="studentTeacherList" ofType="StudentTeacher"><result column="s_id" property="sid"></result><result column="t_id" property="tid"></result><association property="teacher" javaType="Teacher"><id column="tid" property="id"></id><result column="tname" property="name"></result></association></collection></resultMap><!--嵌套查询--><select id="getStudentById2" parameterType="int" resultMap="getStudentMap2">select * from student where sid=#{id}</select><resultMap id="getStudentMap2" type="Student"><id column="sid" property="id"></id><result column="sname" property="name"></result><collection property="studentTeacherList" ofType="StudentTeacher" column="sid"select="com.jn.dao.StudentTeacherDao.getStudentTeacherBySid"></collection></resultMap></mapper>

3.7测试方法

    //多对多的嵌套测试@Testpublic void testGetStudentById2()throws Exception{SqlSession sqlSession = MyBatisUtils.getSession();StudentDao studentDao = sqlSession.getMapper(StudentDao.class);Student student = studentDao.getStudentById2(1);System.out.println(student.getName());List<StudentTeacher> studentTeacherList = student.getStudentTeacherList();studentTeacherList.forEach(System.out::println);sqlSession.close();}

3.8测试结果

九、延迟加载策略

(一)简介

1.什么是延迟加载?

        延迟加载(lazy load)是(也称为懒加载)关联关系对象默认的加载方式，延迟加载机制是为了避免一些无谓的性能开销而提出来的，所谓延迟加载就是当在真正需要数据的时候，才真正执行数据加载操作。
        延迟加载，可以简单理解为，只有在使用的时候，才会发出sql语句进行查询。

2.为什么要使用延迟加载?

        减少访问数据库的频率，我们要访问的数据量过大时，明显用缓存不太合适，因为内存容量有限为了减少并发量，减少系统资源的消耗。

(二)局部延时加载

        注意：只有在嵌套查询的时候才能用到延时加载

        在mybatis中使用resultMap来实现一对一，一对多，多对多关系的操作。主要是通过 association、collection 实现一对一及一对多映射。association、collection 具备延迟加载功能。

 1.现象演示

在进行查询上述Employee与Department关联信息的时候，正常查询结果：

显示了两条sql语句的查询

然后把department.getEmps().forEach(System.out::println);给注释了查看结果

还是查询了两条sql语句

 2.局部解决

</resultMap>
相关联的查询标签上加 fetchType=”lazy”
fetchType默认值为eager 立即加载,Lazy为延时加载。

然后查看结果：

        发现只执行了一条sql语句

然后不进行注释查看结果 ：

正常执行

（三）全局延时加载

        如果希望所有关联都需要延时加载，可以在mybatis的核心配置文件中进行配置，不用在collection或association中指定。默认全局开启。

1.配置setting

<settings>
<!--开启延时加载开关-->
<setting name="lazyLoadingEnabled" value="true"/>
<!--关闭立即加载，实施按需加载-->
<setting name="aggressiveLazyLoading" value="false"/>
</settings>

2.测试 

测试正常