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🥭本文内容:mybatis 多表关联查询
一对多关联查询是指在查询一方对象的时候,同时将其所关联的多方对象也都查询出来。下面以班级 Classes 与学生 Student 间的一对多关系为例进行演示。一个班级有多个学生,一个学生只属于一个班级。数据库 student 表里面有个字段 classno 是外键,对应主键表 Class 的主键 cid。
项目案例:查询班级号为 1801 的班级,同时遍历该班级的所有的学生信息
实现步骤:
【1】在 MySQL 中创建数据库 studentdb,创建表 student 和classes,并添加若干测试用的数据记录,SQL 语句如下:
CREATE DATABASE studentdb;USE studentdb ;DROP TABLE IF EXISTS student ;CREATE TABLE student ( id INT(11) NOT NULL, studentname VARCHAR(20) DEFAULT NULL, gender CHAR(2) DEFAULT NULL, age INT(11) DEFAULT NULL, classno VARCHAR(10), PRIMARY KEY ( id )) INSERT INTO student ( id , studentname , gender , age , classno ) VALUES (1,'张飞','男',18,'201801'),(2,'李白','男',20,'201801'),(3,'张无忌','男',19,'201801'),(4,'赵敏','女',17,'201801');CREATE TABLE classes (cid VARCHAR (30),cname VARCHAR (60)); INSERT INTO classes (cid, cname) VALUES('201801','计算机软件1班');INSERT INTO classes (cid, cname) VALUES('201802','计算机软件2班');
【2】创建实体类 Classes 和 Student 类
Student 类如下:
package cn.kGC.my01.entity;import lombok.Data;@Datapublic class Student { private String sid; private String sname; private String sex; private Integer age; //添加额外属性:所在班级 private Classes classes; public String show(){ return "学生编号:"+getSid()+",学生姓名:"+getSname()+",学生性别:"+getSex()+",学生年龄:"+getAge(); }}
Classes 类如下:
package cn.kgc.my01.entity;import lombok.Data;import java.util.List;@Datapublic class Classes { private String cid; private String cname; //添加额外属性 private List<Student> students; public String show(){ return "班级编号:"+getCid()+",班级名称:"+getCname()+",班级学生:"; }}
【3】创建 ClassesMapper.java 接口,并添加 findClassesById 方法
package cn.kgc.my01.mapper;import cn.kgc.my01.entity.Classes;public interface ClassesMapper { Classes findClassesById(String id);}
【4】创建 ClassesMapper.xml 映射文件,有以下两种方式:
方式一:多表连接查询方式。
这种方式只用到1条 SQL 语句,代码如下所示:
<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "https://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="cn.kgc.my01.mapper.ClassesMapper"> <!--方式一:多表连接查询方式,只用到 1条sql语句--> <resultMap id="classResultMap" type="classes"> <id property="cid" column="cid"/> <result property="cname" column="cname"/> <!--关联属性的映射关系--> <collection property="students" ofType="Student"> <id property="sid" column="id"/> <result property="sname" column="studentname"/> <result property="sex" column="gender"/> <result property="age" column="age"/> </collection> </resultMap> <select id="findClassesById" resultMap="classResultMap"> select cid,cname,id,studentname,gender,age from classes,student where classes.cid=student.classno and classes.cid=#{cid} </select></mapper>
注意: 在
另外,在 “一方” 的映射文件中使用
方式二:多表单独查询方式。
多表连接查询方式是将多张表进行连接,连为一张表后进行查询。其查询的本质是一张表。而多表单独查询方式是多张表各自查询各自的相关内容,需要多张表的联合数据,再将主表的查询结果联合其它表的查询结果,封装为一个对象。
多个查询是可以跨越多个映射文件的,即是可以跨越多个namespace 的。在使用其它 namespace 的查询时,添加上其所在的 namespace 即可。这种方式要用到2条 SQL 语句,代码如下所示:
<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "Https://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="cn.kgc.my01.mapper.ClassesMapper"> <!--方式二:多表单独查询方式,也就是分步查询--> <resultMap id="classResultMap2" type="classes"> <id property="cid" column="cid"/> <result property="cname" column="cname"/> <!--关联属性的映射关系--> <collection property="students" ofType="Student"> <id property="sid" column="id"/> <result property="sname" column="studentname"/> <result property="sex" column="gender"/> <result property="age" column="age"/> </collection> </resultMap> <!-- 以下注释部分属于方式二: 多表单独查询方式 --> <resultMap id="studentResultMap" type="student"> <id property="sid" column="id" /> <result property="sname" column="studentname" /> <result property="sex" column="gender" /> <result property="age" column="age" /> </resultMap> <resultMap id="classesResultMap" type="classes"> <id property="cid" column="cid" /> <result property="cname" column="cname" /> <!-- 关联属性的映射关系 --> <!-- 集合的数据来自指定的select查询,该select查询的动态参数来自column指定的字段值 --> <collection property="students" ofType="Student" select="selectStudentsByClasses" column="cid"/> </resultMap> <!-- 多表单独查询,查多方的表 --> <select id="selectStudentsByClasses" resultMap="studentResultMap"> select * from student where calssno=#{cid} </select> <!-- 多表单独查询,查一方的表 --> <select id="findClassesById" parameterType="String" resultMap="classesResultMap"> select cid,cname from classes where cid=#{cid} </select></mapper>
【5】创建 ClassesMapperTest 测试类,并添加如下方法:
package cn.kgc.my01.mapper;import cn.kgc.my01.entity.Classes;import cn.kgc.my01.entity.Student;import junit.framework.TestCase;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;import org.junit.Before;import org.junit.Test;import java.io.IOException;import java.io.InputStream;import java.util.List;public class ClassesMapperTest{ SqlSessionFactory factory=null; @Before public void init(){ try { System.out.println("########"); InputStream resourceAsStream = Resources.getResourceAsStream("config/mybatis-config.xml"); factory = new SqlSessionFactoryBuilder().build(resourceAsStream); } catch (IOException e) { e.printStackTrace(); } } @Test public void testFindClassesById() { SqlSession sqlSession = factory.openSession(true); ClassesMapper mapper = sqlSession.getMapper(ClassesMapper.class); Classes classesById = mapper.findClassesById("201801"); System.out.println(classesById.show()); List<Student> students = classesById.getStudents(); for (Student student : students) { System.out.println(student.show()); } }}
方式一:多表连接查询方式测试结果:
DEBUG [main] - ==> Preparing: select cid,cname,id,studentname,gender,age from classes,student where classes.cid=student.classno and classes.cid=?DEBUG [main] - ==> Parameters: 201801(String)DEBUG [main] - <== Total: 4班级编号:201801,班级名称:计算机软件1班,班级学生:学生编号:1,学生姓名:张飞,学生性别:男,学生年龄:18学生编号:2,学生姓名:李白,学生性别:男,学生年龄:20学生编号:3,学生姓名:张无忌,学生性别:男,学生年龄:19学生编号:4,学生姓名:赵敏,学生性别:女,学生年龄:17
可以发现,只有一条 SQL 语句,并且是多表联查。
方式二:多表单独查询方式测试结果:
2023-02-15 10:56:49,965 [main] DEBUG DEBUG [main] - ==> Preparing: select cid,cname from classes where cid=?DEBUG [main] - ==> Parameters: 201801(String)DEBUG [main] - ====> Preparing: select * from student where classno=?DEBUG [main] - ====> Parameters: 201801(String)DEBUG [main] - <==== Total: 4DEBUG [main] - <== Total: 1班级编号:201801,班级名称:计算机软件1班,班级学生:学生编号:1,学生姓名:张飞,学生性别:男,学生年龄:18学生编号:2,学生姓名:李白,学生性别:男,学生年龄:20学生编号:3,学生姓名:张无忌,学生性别:男,学生年龄:19学生编号:4,学生姓名:赵敏,学生性别:女,学生年龄:17
可以发现,其 SQL 语句是两条,即各查各的,共用同一个参数。第 1 条先查一方的表,第 2 条再查多方的表。
多对一关联查询是指在查询多方对象的时候,同时将其所关联的一方对象也查询出来。
由于在查询多方对象时也是一个一个查询,所以多对一关联查询,其实就是一对一关联查询。即一对一关联查询的实现方式与多对一的实现方式是相同的。 配置多对一关联的重点在于“多方”的映射文件要有
项目案例: 查询学号为1的学生,同时获取他所在班级的完整信息
实现步骤:
【1】创建 StudentMapper.java 接口,并添加方法 searchStudentsById(int id) 如下:
package cn.kgc.my01.mapper;import cn.kgc.my01.entity.Student;public interface StudentMapper { public Student searchStudentsById(int id);}
【2】创建 StudentMapper.xml 映射文件,有以下两种方式:
方式一:多表联合查询。
<!-- 多表联合查询 --> <resultMap id="studentResultMapper" type="student"> <id property="sid" column="id" /> <result property="sname" column="studentname" /> <result property="sex" column="gender" /> <result property="age" column="age" /> <!-- 关联属性 --> <association property="classes" javaType="classes"> <id property="cid" column="cid" /> <result property="cname" column="cname" /> </association> </resultMap> <!-- 多表连接查询 --> <select id="searchStudentsById" parameterType="int" resultMap="studentResultMapper"> select cid,cname,id,studentname,gender,age from classes,student where classes.cid=student.classno and student.id=#{id} </select>
方式二:多表单独查询。
<!-- 以下注释的是方式二:多表单独查询 --> <resultMap id="studentResultMap2" type="student"> <id property="sid" column="id" /> <result property="sname" column="studentname" /> <result property="sex" column="gender" /> <result property="age" column="age" /> <!-- 关联属性 --> <association property="classes" javaType="classes" select="findClassesById" column="classno"/> </resultMap> <select id="searchStudentsById" resultMap="studentResultMap2"> select id,studentname,gender,age,classno from student where id=#{id} </select> <select id="findClassesById" parameterType="String" resultType="classes"> select cid,cname from classes where cid=#{cid} </select>
【3】创建 StudentMapperTest 测试类,并添加如下方法:
package cn.kgc.my01.mapper;import cn.kgc.my01.entity.Classes;import cn.kgc.my01.entity.Student;import junit.framework.TestCase;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;import org.junit.Before;import org.junit.Test;import java.io.IOException;import java.io.InputStream;public class StudentMapperTest { SqlSessionFactory factory=null; @Before public void init(){ try { InputStream resourceAsStream = Resources.getResourceAsStream("config/mybatis-config.xml"); factory = new SqlSessionFactoryBuilder().build(resourceAsStream); } catch (IOException e) { e.printStackTrace(); } } @Test public void testSearchStudentsById() { SqlSession sqlSession = factory.openSession(true); StudentMapper mapper = sqlSession.getMapper(StudentMapper.class); Student student = mapper.searchStudentsById(1); System.out.println(student.show()); System.out.println("所在班级:"); Classes classes=student.getClasses(); System.out.println(classes.toString()); }}
方式一:多表联合查询方式测试结果:
DEBUG [main] - ==> Preparing: select cid,cname,id,studentname,gender,age from classes,student where classes.cid=student.classno and student.id=?DEBUG [main] - ==> Parameters: 1(Integer)DEBUG [main] - <== Total: 1学生编号:1,学生姓名:张飞,学生性别:男,学生年龄:18所在班级:Classes(cid=201801, cname=计算机软件1班, students=null)
可以发现,它发出的 SQL 语句是多表查询。
方式一:多表单独查询方式测试结果:
DEBUG [main] - ==> Preparing: select id,studentname,gender,age,classno from student where id=?DEBUG [main] - ==> Parameters: 1(Integer)DEBUG [main] - ====> Preparing: select cid,cname from classes where cid=?DEBUG [main] - ====> Parameters: 201801(String)DEBUG [main] - <==== Total: 1DEBUG [main] - <== Total: 1学生编号:1,学生姓名:张飞,学生性别:男,学生年龄:18所在班级:Classes(cid=201801, cname=计算机软件1班, students=null)
可以发现,它发出的 SQL 语句是两条,即各查各的,共用同一个参数。
自连接的查询可以用一对多来处理,也可以用多对一来处理。例如,员工表,每个员工都有一个上司,但上司同时也是员工表的一条记录,这种情况可用自连接查询出每个员工对应的上司信息,也可以查出每个上司有哪些下属员工。
项目案例:查询员工的信息及对应的上司信息。
思路分析: 可将员工当做多方,上司当做一方。
实现步骤:
【1】修改数据库。
添加一个表 employee 并插入测试数据,具体如下:
create table employee (empid double ,empname varchar (60),job varchar (60),leader double ); insert into employee (empid, empname, job, leader) values('1','jack','clerk','3');insert into employee (empid, empname, job, leader) values('2','mike','salesman','3');insert into employee (empid, empname, job, leader) values('3','john','manager','4');insert into employee (empid, empname, job, leader) values('4','smith','president',NULL);insert into employee (empid, empname, job, leader) values('5','rose','salesman','3');
【2】创建实体类 Employee,代码如下:
package cn.kgc.my01.entity;public class Employee {private int empid;private String empname;private String job;private Employee leader;public int getEmpid() {return empid;}public void setEmpid(int empid) {this.empid = empid;}public String getEmpname() {return empname;}public void setEmpname(String empname) {this.empname = empname;}public String getJob() {return job;}public void setJob(String job) {this.job = job;}public Employee getLeader() {return leader;}public void setLeader(Employee leader) {this.leader = leader;}public String toString(){return "员工编号:"+getEmpid()+",员工姓名:"+getEmpname()+",员工职位:"+getJob();}}
可以发现,里面存在着嵌套,Employee 里面的一个属性 leader 本身就是 Employee 类型。
【3】创建 EmployeeMapper.java 接口,添加 findEmployeeAndLeaderById 方法如下:
package cn.kgc.my01.mapper;import cn.kgc.my01.entity.Employee;public interface EmployeeMapper { Employee findEmployeeAndLeaderById(int id);}
【4】创建 EmployeeMapper.xml 映射文件,代码如下:
<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "https://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="cn.kgc.my01.mapper.EmployeeMapper"> <resultMap id="empResultMap" type="employee"> <id property="empid" column="empid" /> <result property="empname" column="empname" /> <result property="job" column="job" /> <association property="leader" javaType="Employee" select="findEmployeeAndLeaderById" column="leader"/> </resultMap> <select id="findEmployeeAndLeaderById" parameterType="int" resultMap="empResultMap"> select * from employee where empid=#{empid} </select></mapper>
【5】创建 EmployeeMapperTest.java 测试类
package cn.kgc.my01.mapper;import cn.kgc.my01.entity.Classes;import cn.kgc.my01.entity.Employee;import cn.kgc.my01.entity.Student;import junit.framework.TestCase;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;import org.junit.Before;import org.junit.Test;import java.io.IOException;import java.io.InputStream;import java.util.Scanner;public class EmployeeMapperTest { SqlSessionFactory factory=null; @Before public void init(){ try { InputStream resourceAsStream = Resources.getResourceAsStream("config/mybatis-config.xml"); factory = new SqlSessionFactoryBuilder().build(resourceAsStream); } catch (IOException e) { e.printStackTrace(); } } @Test public void testFindEmployeeAndLeaderById() { SqlSession sqlSession = factory.openSession(true); EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class); Employee employee=mapper.findEmployeeAndLeaderById(1); Employee leader=employee.getLeader(); System.out.println(employee.toString()); System.out.println("他的上司是:"+leader.toString()); //System.out.println("他的上司的上司是:"+leader.getLeader().toString()); }}
测试结果: 查询员工
DEBUG [main] - ==> Preparing: select * from employee where empid=?DEBUG [main] - ==> Parameters: 1(Integer)DEBUG [main] - ====> Preparing: select * from employee where empid=?DEBUG [main] - ====> Parameters: 3(Integer)DEBUG [main] - ======> Preparing: select * from employee where empid=?DEBUG [main] - ======> Parameters: 4(Integer)DEBUG [main] - <====== Total: 1DEBUG [main] - <==== Total: 1DEBUG [main] - <== Total: 1员工编号:1,员工姓名:jack,员工职位:clerk他的上司是:员工编号:3,员工姓名:john,员工职位:manager
从上面的 SQL 语句中发现,出现了 3 条 SQL 语句,这个查询存在嵌套,先查员工1,然后查他的直接上司3,再查上司的上司4。这种情况不影响什么,甚至可以实现直接输出上司的上司,但要注意输出语句不要出现地柜,即输出语句不要出现输出上司。
要同时查上司的上司,只需要在上面的测试类中多加一条语句:
System.out.println("他的上司的上司是:"+leader.getLeader().toString());
项目案例:查询某位领导及其直接下属员工。
思路分析: 可用一对多的方式来实现,员工(领导)当作一方,员工(下属)当作多方。
实现步骤:
【1】修改实体类 Employee,代码如下:
package cn.kgc.my01.entity;import java.util.List;public class Employee {private int empid;private String empname;private String job;//员工的上司private Employee leader;//员工的下属private List<Employee> employees;public List<Employee> getEmployees() {return employees;}public void setEmployees(List<Employee> employees) {this.employees = employees;}public int getEmpid() {return empid;}public void setEmpid(int empid) {this.empid = empid;}public String getEmpname() {return empname;}public void setEmpname(String empname) {this.empname = empname;}public String getJob() {return job;}public void setJob(String job) {this.job = job;}public Employee getLeader() {return leader;}public void setLeader(Employee leader) {this.leader = leader;}public String toString(){return "员工编号:"+getEmpid()+",员工姓名:"+getEmpname()+",员工职位:"+getJob();}}
【2】在 EmployeeMapper.java 接口中,添加 findLeaderAndEmployeesById 方法如下:
Employee findLeaderAndEmployeesById(int id);
【3】在 EmployeeMapper.xml 映射文件中,添加 findEmployeeAndLeaderById 的映射方法内容如下:
<!-- 一对多的方式实现自连接 --> <resultMap id="empResultMap2" type="employee"> <id property="empid" column="empid" /> <result property="empname" column="empname" /> <result property="job" column="job" /> <!-- 关联属性的映射关系 集合的数据来自指定的select查询,该select查询的动态参数来自column指定的字段值 --> <collection property="employees" ofType="employee" select="selectEmployeesByLeader" column="empid"/> </resultMap> <select id="selectEmployeesByLeader" resultType="employee"> select * from employee where leader=#{empid} </select> <select id="findLeaderAndEmployeesById" parameterType="int" resultMap="empResultMap2"> select * from employee where empid=#{empid} </select>
【4】在 EmployeeMapperTest.java 测试类中,添加如下内容:
@Test public void testTestFindLeaderAndEmployeesById() { SqlSession sqlSession = factory.openSession(true); EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class); Employee leader=mapper.findLeaderAndEmployeesById(4); List<Employee> employees = leader.getEmployees(); System.out.println(leader.toString()); System.out.println("他的直接下属有:"); for (Employee employee : employees) { System.out.println(employee.toString()); } }
测试结果: 查询经理
DEBUG [main] - ==> Preparing: select * from employee where empid=?DEBUG [main] - ==> Parameters: 4(Integer)DEBUG [main] - ====> Preparing: select * from employee where leader=?DEBUG [main] - ====> Parameters: 4.0(Double)DEBUG [main] - <==== Total: 1DEBUG [main] - <== Total: 1员工编号:4,员工姓名:smith,员工职位:president他的直接下属有:员工编号:3,员工姓名:john,员工职位:manager
原理: 多对多可以分拆成两个一对多来处理,需要一个中间表,各自与中间表实现一对多的关系。
项目案例:一个学生可以选人修多门课程,一门课程可以给多个学生选修,课程与学生之间是典型的多对多。实现查询一个学生信息,同时查出他的所有选修课,还有实现查询一门课程信息,同时查出所有的选修了该课程的学生信息。
思路分析: 多对多需要第三表来体现,数据库中除了课程表,学生表,还需要学生课程表。
实现步骤:
【1】修改数据库,代码如下:
CREATE DATABASE studentdb;USE studentdb ;DROP TABLE IF EXISTS student ;CREATE TABLE student ( id INT(11) NOT NULL, studentname VARCHAR(20) DEFAULT NULL, gender CHAR(2) DEFAULT NULL, age INT(11) DEFAULT NULL, classno VARCHAR(10), PRIMARY KEY ( id )) INSERT INTO student ( id , studentname , gender , age , classno ) VALUES (1,'张飞','男',18,'201801'),(2,'李白','男',20,'201801'),(3,'张无忌','男',19,'201801'),(4,'赵敏','女',17,'201801');CREATE TABLE classes (cid VARCHAR (30),cname VARCHAR (60)); INSERT INTO classes (cid, cname) VALUES('201801','计算机软件1班');INSERT INTO classes (cid, cname) VALUES('201802','计算机软件2班');CREATE TABLE employee (empid DOUBLE ,empname VARCHAR (60),job VARCHAR (60),leader DOUBLE ); INSERT INTO employee (empid, empname, job, leader) VALUES('1','jack','clerk','3');INSERT INTO employee (empid, empname, job, leader) VALUES('2','mike','salesman','3');INSERT INTO employee (empid, empname, job, leader) VALUES('3','john','manager','4');INSERT INTO employee (empid, empname, job, leader) VALUES('4','smith','president',NULL);INSERT INTO employee (empid, empname, job, leader) VALUES('5','rose','salesman','3');CREATE TABLE course (courseid DOUBLE ,coursename VARCHAR (90)); INSERT INTO course (courseid, coursename) VALUES('1','java');INSERT INTO course (courseid, coursename) VALUES('2','Android');INSERT INTO course (courseid, coursename) VALUES('3','PHP');CREATE TABLE studentcourse (id DOUBLE ,studentid DOUBLE ,courseid DOUBLE ); INSERT INTO studentcourse (id, studentid, courseid) VALUES('1','1','1');INSERT INTO studentcourse (id, studentid, courseid) VALUES('2','1','2');INSERT INTO studentcourse (id, studentid, courseid) VALUES('3','2','1');INSERT INTO studentcourse (id, studentid, courseid) VALUES('4','2','2');INSERT INTO studentcourse (id, studentid, courseid) VALUES('5','3','1');INSERT INTO studentcourse (id, studentid, courseid) VALUES('6','3','2');INSERT INTO studentcourse (id, studentid, courseid) VALUES('7','1','3');
【2】新增实体类 Course 和修改实体类 Student 。
Course 类如下:
package cn.kgc.my01.entity;import lombok.Data;import java.util.List;@Datapublic class Course { private int courseid; private String coursename; private List<Student> students; public String toString(){ return "课程编号:"+getCourseid()+",课程名称:"+getCoursename(); }}
Student类如下,添加一个属性courses和getter,setter方法。
private List<Course> courses; public List<Course> getCourses() { return courses; } public void setCourses(List<Course> courses) { this.courses = courses; }
【3】新建 StudentMapper.java 接口,并添加一个方法如下:
package cn.kgc.my01.mapper;import cn.kgc.my01.entity.Student;public interface StudentMapper { public Student searchStudentById(int id);}
【4】配置对应的 StudentMapper.xml 映射,代码如下:
<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "https://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="cn.kgc.my01.mapper.StudentMapper"> <resultMap id="studentMap2" type="student"> <id property="sid" column="id" /> <result property="sname" column="studentname" /> <result property="sex" column="gender" /> <result property="age" column="age" /> <!-- 关联属性的映射关系 --> <collection property="courses" ofType="Course"> <id property="courseid" column="courseid" /> <result property="coursename" column="coursename" /> </collection> </resultMap> <!-- 多表连接查询 --> <select id="searchStudentById" parameterType="int" resultMap="studentMap2"> select student.id,studentname,gender,age,course.courseid,coursename from course,student,studentcourse where course.courseid=studentcourse.courseid and student.id=studentcourse.studentid and student.id=#{id} </select></mapper>
【5】创建测试类 StudentMapperTest 类
package cn.kgc.my01.mapper;import cn.kgc.my01.entity.Classes;import cn.kgc.my01.entity.Course;import cn.kgc.my01.entity.Student;import junit.framework.TestCase;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;import org.junit.Before;import org.junit.Test;import java.io.IOException;import java.io.InputStream;import java.util.List;public class StudentMapperTest { SqlSessionFactory factory=null; @Before public void init(){ try { InputStream resourceAsStream = Resources.getResourceAsStream("config/mybatis-config.xml"); factory = new SqlSessionFactoryBuilder().build(resourceAsStream); } catch (IOException e) { e.printStackTrace(); } } @Test public void testSearchStudentsById() { SqlSession sqlSession = factory.openSession(true); StudentMapper mapper = sqlSession.getMapper(StudentMapper.class); Student student = mapper.searchStudentById(1); System.out.println(student.show()); System.out.println("-----该生选修了以下课程:-----------"); List<Course> courses=student.getCourses(); for(Course course:courses){ System.out.println(course.toString()); } }}
测试结果:
【6】新建 CourseMapper.java 接口,并添加一个方法如下:
package cn.kgc.my01.mapper;import cn.kgc.my01.entity.Course;public interface CourseMapper { //根据id查找课程,即时获取选个性该课程的学生 public Course searchCourseById(int id);}
【7】配置对应的 CourseMapper.xml 映射,代码如下:
<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "https://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="cn.kgc.my01.mapper.CourseMapper"> <resultMap id="courseMap" type="course"> <id property="courseid" column="courseid" /> <result property="coursename" column="coursename" /> <!-- 关联属性的映射关系 --> <collection property="students" ofType="Student"> <id property="sid" column="id" /> <result property="sname" column="studentname" /> <result property="sex" column="gender" /> <result property="age" column="age" /> </collection> </resultMap> <!-- 多表连接查询 --> <select id="searchCourseById" parameterType="int" resultMap="courseMap"> select student.id,studentname,gender,age,course.courseid,coursename from course,student,studentcourse where course.courseid=studentcourse.courseid and student.id=studentcourse.studentid and course.courseid=#{courseid} </select></mapper>
【8】创建测试类 CourseMapperTest 类
package cn.kgc.my01.mapper;import cn.kgc.my01.entity.Course;import cn.kgc.my01.entity.Employee;import cn.kgc.my01.entity.Student;import junit.framework.TestCase;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;import org.junit.Before;import org.junit.Test;import java.io.IOException;import java.io.InputStream;import java.util.List;public class CourseMapperTest{ SqlSessionFactory factory=null; @Before public void init(){ try { InputStream resourceAsStream = Resources.getResourceAsStream("config/mybatis-config.xml"); factory = new SqlSessionFactoryBuilder().build(resourceAsStream); } catch (IOException e) { e.printStackTrace(); } } @Test public void testSearchCourseById() { SqlSession sqlSession = factory.openSession(true); CourseMapper mapper = sqlSession.getMapper(CourseMapper.class); Course course = mapper.searchCourseById(1); System.out.println(course.toString()); System.out.println("-------该课程有以下学生选修:------"); List<Student> students=course.getStudents(); for(Student student:students){ System.out.println(student.show()); } }}
测试效果:
码文不易,本篇文章就介绍到这里,如果想要学习更多Java系列知识,点击关注博主,博主带你零基础学习Java知识。与此同时,对于日常生活有困扰的朋友,欢迎阅读我的第四栏目:《国学周更—心性养成之路》,学习技术的同时,我们也注重了心性的养成。
来源地址:https://blog.csdn.net/hh867308122/article/details/129005131
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