七、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
java
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;
}
@Override
public String toString() {
return "Person{" +
"id=" + id +
", name='" + name + '\'' +
'}';
}
}IdCard
java
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;
}
@Override
public 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
java
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
java
package com.jn.dao;
import com.jn.entity.PersonIdCard;
public interface PersonDao {
public PersonIdCard getPersonById(int id);
}3.5定义 PersonDao.xml
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.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
java
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 {
@Test
public 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);
//commit
sqlSession.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
java
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;
}
@Override
public String toString() {
return "Person{" +
"id=" + id +
", name='" + name + '\'' +
", idCard=" + idCard +
'}';
}
}4.2持久层里面添加方法
java
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
java
package com.jn.dao;
import com.jn.entity.Person;
public interface IdCardDao {
public Person getIdCardByPersonId(Integer id);
}4.4定义 IdCardDao.xml
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.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
<?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.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
java
//测试嵌套查询
@Test
public 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
java
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
java
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
java
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;
}
@Override
public String toString() {
return "Employee{" +
"id=" + id +
", name='" + name + '\'' +
", gender='" + gender + '\'' +
", age=" + age +
'}';
}
}2.2编写sql查询语句
java
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
java
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;
}
@Override
public String toString() {
return "Department{" +
"id=" + id +
", name='" + name + '\'' +
", emps=" + emps +
'}';
}
}2.4持久层DepartmentDao
java
package com.jn.dao;
import com.jn.entity.Department;
public interface DepartmentDao {
public Department getDepartById(Integer id);
}2.5映射文件
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.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
java
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 {
@Test
public 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);
//close
sqlSession.close();
}
}2.7测试结果
3.嵌套查询的方式
3.1定义EmployeeDao
java
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
<?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.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
java
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
<?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.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
java
//嵌套查询
@Test
public 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);
//close
sqlSession.close();
}3.6测试结果
(三)MyBatis实现多对多查询
1.创建数据库表
bash
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
java
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;
}
}
java
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;
}
@Override
public String toString() {
return "Teacher{" +
"id=" + id +
", name='" + name + '\'' +
'}';
}
}
java
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
java
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
java
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;
}
@Override
public String toString() {
return "Student{" +
"id=" + id +
", name=" + name +
", studentTeacherList=" + studentTeacherList +
'}';
}
}2.4ST加入Teacher属性
java
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;
}
@Override
public String toString() {
return "StudentTeacher{" +
"sid=" + sid +
", tid=" + tid +
", teacher=" + teacher +
'}';
}
}2.5tudentDao编写
java
package com.jn.dao;
import com.jn.entity.Student;
public interface StudentDao {
public Student getStudentById(Integer id);
}2.6SudentDao.xml
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.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
java
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 {
@Test
public 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
java
package com.jn.dao;
import com.jn.entity.Teacher;
public interface TeacherDao {
public Teacher getTeacherById(Integer id);
}3.2TeacherDao.xml
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.jn.dao.TeacherDao">
<select id="getTeacherById" parameterType="int" resultType="Teacher">
select tid id,tname name from teacher where tid =#{id}
</select>
</mapper>3.3StudentTeacherDao
java
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
<?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.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
java
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
<?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.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
java
//多对多的嵌套测试
@Test
public 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
八、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
XML
<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
