个人可以做淘宝客网站吗,黑龙江网站建设巨耀网络,wordpress添加登入,餐饮品牌全案策划公司八、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#xff0c;对于复杂业务来讲#xff0c;关联的表有几张#xff0c;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#xff0c;就必须进行多表查询#xff0c…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_idc.c_person_id and p.p_id1; 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 version1.0 encodingUTF-8 ?
!DOCTYPE mapperPUBLIC -//mybatis.org//DTD Mapper 3.0//ENhttp://mybatis.org/dtd/mybatis-3-mapper.dtd
mapper namespacecom.jn.dao.PersonDaoselect idgetPersonById resultMapPersonResultMapSELECT p.*,c.* from person p ,idcard c where p.p_id c.c_person_id and p.p_id 1;/selectresultMap idPersonResultMap typePersonIdCardid columnp_id propertyid/idresult columnp_id propertyid/resultresult columnp_name propertyname/resultresult columnc_cardno propertycardno/resultresult columnc_uselife propertyuseLife/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 version1.0 encodingUTF-8 ?
!DOCTYPE mapperPUBLIC -//mybatis.org//DTD Mapper 3.0//ENhttp://mybatis.org/dtd/mybatis-3-mapper.dtd
mapper namespacecom.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 idgetIdCardByPersonId parameterTypeint resultMapIdCardResultselect * from idcard where c_person_id #{id}/selectresultMap idIdCardResult typeIdCardid columnc_id propertyid/idresult columnc_id propertyid/resultresult columnc_cardno propertycardno/resultresult columnc_uselife propertyuseLife/result/resultMap
/mapper
4.5PersonDao.xml 配置
?xml version1.0 encodingUTF-8 ?
!DOCTYPE mapperPUBLIC -//mybatis.org//DTD Mapper 3.0//ENhttp://mybatis.org/dtd/mybatis-3-mapper.dtd
mapper namespacecom.jn.dao.PersonDao!--嵌套结果的查询--select idgetPersonById resultMapPersonResultMapSELECT p.*,c.* from person p ,idcard c where p.p_id c.c_person_id and p.p_id 1;/selectresultMap idPersonResultMap typePersonIdCardid columnp_id propertyid/idresult columnp_id propertyid/resultresult columnp_name propertyname/resultresult columnc_cardno propertycardno/resultresult columnc_uselife propertyuseLife/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 idgetPersonById2 parameterTypeint resultMapPersonResultMap2select * from person where p_id #{id}/selectresultMap idPersonResultMap2 typePersonid columnp_id propertyid/idresult columnp_name propertyname/result!--映射Person的复杂字段idCard对象属性--association propertyidCard javaTypeIdCard columnp_id selectcom.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_ide.e_depart_id and d.d_id1; 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 ListEmployee 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 ListEmployee getEmps() {return emps;}public void setEmps(ListEmployee 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 version1.0 encodingUTF-8 ?
!DOCTYPE mapperPUBLIC -//mybatis.org//DTD Mapper 3.0//ENhttp://mybatis.org/dtd/mybatis-3-mapper.dtd
mapper namespacecom.jn.dao.DepartmentDaoselect idgetDepartById parameterTypeint resultMapgetDepartmentMapselect d.*,e.* from department d,employee e where d.d_ide.e_depart_id and d.d_id#{id}/selectresultMap idgetDepartmentMap typeDepartmentid columnd_id propertyid/idresult columnd_name propertyname/resultcollection propertyemps ofTypeEmployeeid columne_id propertyid/idresult columne_name propertyname/resultresult columne_gender propertygender/resultresult columne_age propertyage/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 ListEmployee getEmployeeById(Integer id);
}3.2定义 EmployeeDao.xml
?xml version1.0 encodingUTF-8 ?
!DOCTYPE mapperPUBLIC -//mybatis.org//DTD Mapper 3.0//ENhttp://mybatis.org/dtd/mybatis-3-mapper.dtd
mapper namespacecom.jn.dao.EmployeeDao!--配置employee的查询语句返回一个Employee对象的集合--select idgetEmployeeById parameterTypeint resultMapgetEmployeeResultMapselect * from employee where e_depart_id #{id}/selectresultMap idgetEmployeeResultMap typeEmployeeid columne_id propertyid/idresult columne_name propertyname/resultresult columne_gender propertygender/resultresult columne_age propertyage/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 version1.0 encodingUTF-8 ?
!DOCTYPE mapperPUBLIC -//mybatis.org//DTD Mapper 3.0//ENhttp://mybatis.org/dtd/mybatis-3-mapper.dtd
mapper namespacecom.jn.dao.DepartmentDaoselect idgetDepartById parameterTypeint resultMapgetDepartmentMapselect d.*,e.* from department d,employee e where d.d_ide.e_depart_id and d.d_id#{id}/selectresultMap idgetDepartmentMap typeDepartmentid columnd_id propertyid/idresult columnd_name propertyname/resultcollection propertyemps ofTypeEmployeeid columne_id propertyid/idresult columne_name propertyname/resultresult columne_gender propertygender/resultresult columne_age propertyage/result/collection/resultMapselect idgetDepartById2 parameterTypeint resultMapgetDepartmentMap2select * from department where d_id#{id}/selectresultMap idgetDepartmentMap2 typeDepartmentid columnd_id propertyid/idresult columnd_name propertyname/resultcollection propertyemps ofTypeEmployee columnd_id selectcom.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_idt.tid AND s.sid1; 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 ListStudentTeacher 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 ListStudentTeacher getStudentTeacherList() {return studentTeacherList;}public void setStudentTeacherList(ListStudentTeacher 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 version1.0 encodingUTF-8 ?
!DOCTYPE mapperPUBLIC -//mybatis.org//DTD Mapper 3.0//ENhttp://mybatis.org/dtd/mybatis-3-mapper.dtd
mapper namespacecom.jn.dao.StudentDaoselect idgetStudentById parameterTypeint resultMapgetStudentMapselect s.*,t.*,st.* from student s,teacher t,student_teacher st where s.sid st.s_id and st.t_idt.tid AND s.sid#{id}/selectresultMap idgetStudentMap typeStudentid columnsid propertyid/idresult columnsname propertyname/resultcollection propertystudentTeacherList ofTypeStudentTeacherresult columns_id propertysid/resultresult columnt_id propertytid/resultassociation propertyteacher javaTypeTeacherid columntid propertyid/idresult columntname propertyname/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());ListStudentTeacher 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 version1.0 encodingUTF-8 ?
!DOCTYPE mapperPUBLIC -//mybatis.org//DTD Mapper 3.0//ENhttp://mybatis.org/dtd/mybatis-3-mapper.dtd
mapper namespacecom.jn.dao.TeacherDaoselect idgetTeacherById parameterTypeint resultTypeTeacherselect 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 ListStudentTeacher getStudentTeacherBySid(Integer id);
}3.4StudentTeacherDao.xml
?xml version1.0 encodingUTF-8 ?
!DOCTYPE mapperPUBLIC -//mybatis.org//DTD Mapper 3.0//ENhttp://mybatis.org/dtd/mybatis-3-mapper.dtd
mapper namespacecom.jn.dao.StudentTeacherDaoselect idgetStudentTeacherBySid parameterTypeint resultMapgetStudentTeacherMapselect * from student_teacher where s_id #{id}/selectresultMap idgetStudentTeacherMap typeStudentTeacherresult columns_id propertysid/resultresult columnt_id propertytid/resultassociation propertyteacher columnt_id javaTypeTeacherselectcom.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 version1.0 encodingUTF-8 ?
!DOCTYPE mapperPUBLIC -//mybatis.org//DTD Mapper 3.0//ENhttp://mybatis.org/dtd/mybatis-3-mapper.dtd
mapper namespacecom.jn.dao.StudentDaoselect idgetStudentById parameterTypeint resultMapgetStudentMapselect s.*,t.*,st.* from student s,teacher t,student_teacher st where s.sid st.s_id and st.t_idt.tid AND s.sid#{id}/selectresultMap idgetStudentMap typeStudentid columnsid propertyid/idresult columnsname propertyname/resultcollection propertystudentTeacherList ofTypeStudentTeacherresult columns_id propertysid/resultresult columnt_id propertytid/resultassociation propertyteacher javaTypeTeacherid columntid propertyid/idresult columntname propertyname/result/association/collection/resultMap!--嵌套查询--select idgetStudentById2 parameterTypeint resultMapgetStudentMap2select * from student where sid#{id}/selectresultMap idgetStudentMap2 typeStudentid columnsid propertyid/idresult columnsname propertyname/resultcollection propertystudentTeacherList ofTypeStudentTeacher columnsidselectcom.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());ListStudentTeacher 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 namelazyLoadingEnabled valuetrue/
!--关闭立即加载实施按需加载--
setting nameaggressiveLazyLoading valuefalse/
/settings 2.测试
测试正常
