MyBatis多表关联查询的实现示例

一对多查询

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() NOT NULL,
   studentname  VARCHAR() DEFAULT NULL,
   gender  CHAR() DEFAULT NULL,
   age  INT() DEFAULT NULL,
   classno VARCHAR(),
   PRIMARY KEY ( id )
) 
INSERT  INTO  student ( id , studentname , gender , age , classno ) VALUES (,'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 (),
	cname VARCHAR ()
); 
INSERT INTO classes (cid, cname) VALUES('','计算机软件1班');
INSERT INTO classes (cid, cname) VALUES('','计算机软件2班');

【2】创建实体类 Classes 和 Student 类

Student 类如下：

 
package cn.kgc.my.entity;
 
import lombok.Data;
 
@Data
public 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.my.entity;
 
import lombok.Data;
 
import java.util.List;
 
@Data
public 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.my.mapper;
 
import cn.kgc.my.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=".0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper.0//EN"
        "https://mybatis.org/dtd/mybatis--mapper.dtd">
<mapper namespace="cn.kgc.my.mapper.ClassesMapper">
    <!--方式一：多表连接查询方式，只用到条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 <resultMap/> 中，如果字段名与属性名相同时，可以在 <resultMap/> 中添加 autoMapping=“true” 来开启自动映射。

另外，在 “一方” 的映射文件中使用 <collection/> 标签体现出两个实体对象间的关联关系。其两个属性的解释如下：

property：指定关联属性，即 Class 类中的集合属性 students。
ofType：集合属性的泛型类型，即 Student。

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=".0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper.0//EN"
        "https://mybatis.org/dtd/mybatis--mapper.dtd">
<mapper namespace="cn.kgc.my.mapper.ClassesMapper">
   <!--方式二：多表单独查询方式，也就是分步查询-->
    <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>
    <!-- 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.my.mapper;
 
import cn.kgc.my.entity.Classes;
import cn.kgc.my.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("");
        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:(String)
DEBUG [main] - <==      Total:
班级编号：,班级名称：计算机软件1班,班级学生：
学生编号：,学生姓名：张飞,学生性别：男,学生年龄：18
学生编号：,学生姓名：李白,学生性别：男,学生年龄：20
学生编号：,学生姓名：张无忌,学生性别：男,学生年龄：19
学生编号：,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：

 
-02-15 10:56:49,965 [main] DEBUG DEBUG [main] - ==>  Preparing: select cid,cname from classes where cid=?
DEBUG [main] - ==> Parameters:(String)
DEBUG [main] - ====>  Preparing: select * from student where classno=?
DEBUG [main] - ====> Parameters:(String)
DEBUG [main] - <====      Total:
DEBUG [main] - <==      Total:
班级编号：,班级名称：计算机软件1班,班级学生：
学生编号：,学生姓名：张飞,学生性别：男,学生年龄：18
学生编号：,学生姓名：李白,学生性别：男,学生年龄：20
学生编号：,学生姓名：张无忌,学生性别：男,学生年龄：19
学生编号：,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。

多对一查询

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 <association> 属性关联“一方”。

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.my.mapper;
 
import cn.kgc.my.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="studentResultMap" 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="studentResultMap">
    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.my.mapper;
 
import cn.kgc.my.entity.Classes;
import cn.kgc.my.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();
        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:(Integer)
DEBUG [main] - <==      Total:
学生编号：,学生姓名：张飞,学生性别：男,学生年龄：18
所在班级:
Classes(cid=, 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:(Integer)
DEBUG [main] - ====>  Preparing: select cid,cname from classes where cid=?
DEBUG [main] - ====> Parameters:(String)
DEBUG [main] - <====      Total:
DEBUG [main] - <==      Total:
学生编号：,学生姓名：张飞,学生性别：男,学生年龄：18
所在班级:
Classes(cid=, 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。

自连接查询

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。

使用多对一的方式实现自连接

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 (),
	job varchar (),
	leader double 
); 
insert into employee (empid, empname, job, leader) values('','jack','clerk','3');
insert into employee (empid, empname, job, leader) values('','mike','salesman','3');
insert into employee (empid, empname, job, leader) values('','john','manager','4');
insert into employee (empid, empname, job, leader) values('','smith','president',NULL);
insert into employee (empid, empname, job, leader) values('','rose','salesman','3');

【2】创建实体类 Employee，代码如下：

 
package cn.kgc.my.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.my.mapper;
 
import cn.kgc.my.entity.Employee;
 
public interface EmployeeMapper {
    Employee findEmployeeAndLeaderById(int id);
}

【4】创建 EmployeeMapper.xml 映射文件，代码如下：

 
<?xml version=".0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper.0//EN"
        "https://mybatis.org/dtd/mybatis--mapper.dtd">
<mapper namespace="cn.kgc.my.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.my.mapper;
 
import cn.kgc.my.entity.Classes;
import cn.kgc.my.entity.Employee;
import cn.kgc.my.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();
        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:(Integer)
DEBUG [main] - ====>  Preparing: select * from employee where empid=?
DEBUG [main] - ====> Parameters:(Integer)
DEBUG [main] - ======>  Preparing: select * from employee where empid=?
DEBUG [main] - ======> Parameters:(Integer)
DEBUG [main] - <======      Total:
DEBUG [main] - <====      Total:
DEBUG [main] - <==      Total:
员工编号:,员工姓名:jack,员工职位:clerk
他的上司是:员工编号:,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());

使用一对多的方式实现自连接

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.my.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="empResultMap" 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="empResultMap">
    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();
    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:(Integer)
DEBUG [main] - ====>  Preparing: select * from employee where leader=?
DEBUG [main] - ====> Parameters:.0(Double)
DEBUG [main] - <====      Total:
DEBUG [main] - <==      Total:
员工编号:,员工姓名:smith,员工职位:president
他的直接下属有：
员工编号:,员工姓名:john,员工职位:manager

多对多查询

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() NOT NULL,
   studentname  VARCHAR() DEFAULT NULL,
   gender  CHAR() DEFAULT NULL,
   age  INT() DEFAULT NULL,
   classno VARCHAR(),
   PRIMARY KEY ( id )
) 
INSERT  INTO  student ( id , studentname , gender , age , classno ) VALUES (,'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 (),
	cname VARCHAR ()
); 
INSERT INTO classes (cid, cname) VALUES('','计算机软件1班');
INSERT INTO classes (cid, cname) VALUES('','计算机软件2班');
 
CREATE TABLE employee (
	empid DOUBLE ,
	empname VARCHAR (),
	job VARCHAR (),
	leader DOUBLE 
); 
INSERT INTO employee (empid, empname, job, leader) VALUES('','jack','clerk','3');
INSERT INTO employee (empid, empname, job, leader) VALUES('','mike','salesman','3');
INSERT INTO employee (empid, empname, job, leader) VALUES('','john','manager','4');
INSERT INTO employee (empid, empname, job, leader) VALUES('','smith','president',NULL);
INSERT INTO employee (empid, empname, job, leader) VALUES('','rose','salesman','3');
 
 
CREATE TABLE course (
	courseid DOUBLE ,
	coursename VARCHAR ()
); 
INSERT INTO course (courseid, coursename) VALUES('','java');
INSERT INTO course (courseid, coursename) VALUES('','android');
INSERT INTO course (courseid, coursename) VALUES('','PHP');
 
CREATE TABLE studentcourse (
	id DOUBLE ,
	studentid DOUBLE ,
	courseid DOUBLE 
); 
INSERT INTO studentcourse (id, studentid, courseid) VALUES('','1','1');
INSERT INTO studentcourse (id, studentid, courseid) VALUES('','1','2');
INSERT INTO studentcourse (id, studentid, courseid) VALUES('','2','1');
INSERT INTO studentcourse (id, studentid, courseid) VALUES('','2','2');
INSERT INTO studentcourse (id, studentid, courseid) VALUES('','3','1');
INSERT INTO studentcourse (id, studentid, courseid) VALUES('','3','2');
INSERT INTO studentcourse (id, studentid, courseid) VALUES('','1','3');

【2】新增实体类 Course 和修改实体类 Student 。

Course 类如下:

 
package cn.kgc.my.entity;
 
import lombok.Data;
 
import java.util.List;
 
@Data
public 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.my.mapper;
 
import cn.kgc.my.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=".0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper.0//EN"
        "https://mybatis.org/dtd/mybatis--mapper.dtd">
<mapper namespace="cn.kgc.my.mapper.StudentMapper">
    <resultMap id="studentMap" 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="studentMap">
        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.my.mapper;
 
import cn.kgc.my.entity.Classes;
import cn.kgc.my.entity.Course;
import cn.kgc.my.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();
        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.my.mapper;
 
import cn.kgc.my.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=".0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper.0//EN"
        "https://mybatis.org/dtd/mybatis--mapper.dtd">
<mapper namespace="cn.kgc.my.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.my.mapper;
 
import cn.kgc.my.entity.Course;
import cn.kgc.my.entity.Employee;
import cn.kgc.my.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();
        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：

	CREATE DATABASE studentdb;
	USE studentdb ;
	DROP TABLE IF EXISTS student ;
	CREATE TABLE student (
	id INT() NOT NULL,
	studentname VARCHAR() DEFAULT NULL,
	gender CHAR() DEFAULT NULL,
	age INT() DEFAULT NULL,
	classno VARCHAR(),
	PRIMARY KEY ( id )
	)
	INSERT INTO student ( id , studentname , gender , age , classno ) VALUES (,'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 (),
	cname VARCHAR ()
	);
	INSERT INTO classes (cid, cname) VALUES('','计算机软件1班');
	INSERT INTO classes (cid, cname) VALUES('','计算机软件2班');

	package cn.kgc.my.entity;

	import lombok.Data;

	@Data
	public 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();
	}
	}

	package cn.kgc.my.entity;

	import lombok.Data;

	import java.util.List;

	@Data
	public 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：";
	}
	}

	package cn.kgc.my.mapper;

	import cn.kgc.my.entity.Classes;

	public interface ClassesMapper {
	Classes findClassesById(String id);
	}

	<?xml version=".0" encoding="UTF-8" ?>
	<!DOCTYPE mapper
	PUBLIC "-//mybatis.org//DTD Mapper.0//EN"
	"https://mybatis.org/dtd/mybatis--mapper.dtd">
	<mapper namespace="cn.kgc.my.mapper.ClassesMapper">
	<!--方式一：多表连接查询方式，只用到条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>

	package cn.kgc.my.mapper;

	import cn.kgc.my.entity.Classes;
	import cn.kgc.my.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("");
	System.out.println(classesById.show());
	List<Student> students = classesById.getStudents();

	for (Student student : students) {
	System.out.println(student.show());
	}
	}
	}

	DEBUG [main] - ==> Preparing: select cid,cname,id,studentname,gender,age from
	classes,student where classes.cid=student.classno and classes.cid=?
	DEBUG [main] - ==> Parameters:(String)
	DEBUG [main] - <== Total:
	班级编号：,班级名称：计算机软件1班,班级学生：
	学生编号：,学生姓名：张飞,学生性别：男,学生年龄：18
	学生编号：,学生姓名：李白,学生性别：男,学生年龄：20
	学生编号：,学生姓名：张无忌,学生性别：男,学生年龄：19
	学生编号：,学生姓名：赵敏,学生性别：女,学生年龄：17

	-02-15 10:56:49,965 [main] DEBUG DEBUG [main] - ==> Preparing: select cid,cname from classes where cid=?
	DEBUG [main] - ==> Parameters:(String)
	DEBUG [main] - ====> Preparing: select * from student where classno=?
	DEBUG [main] - ====> Parameters:(String)
	DEBUG [main] - <==== Total:
	DEBUG [main] - <== Total:
	班级编号：,班级名称：计算机软件1班,班级学生：
	学生编号：,学生姓名：张飞,学生性别：男,学生年龄：18
	学生编号：,学生姓名：李白,学生性别：男,学生年龄：20
	学生编号：,学生姓名：张无忌,学生性别：男,学生年龄：19
	学生编号：,学生姓名：赵敏,学生性别：女,学生年龄：17

	package cn.kgc.my.mapper;

	import cn.kgc.my.entity.Student;

	public interface StudentMapper {
	public Student searchStudentsById(int id);
	}

	<!-- 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>

	<!-- 以下注释的是方式二：多表单独查询 -->
	<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" />
	<!-- 关联属性 -->
	<association property="classes" javaType="classes" select="findClassesById" column="classno"/>
	</resultMap>
	<select id="searchStudentsById" resultMap="studentResultMap">
	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>

	DEBUG [main] - ==> Preparing: select cid,cname,id,studentname,gender,age from
	classes,student where classes.cid=student.classno and student.id=?
	DEBUG [main] - ==> Parameters:(Integer)
	DEBUG [main] - <== Total:
	学生编号：,学生姓名：张飞,学生性别：男,学生年龄：18
	所在班级:
	Classes(cid=, cname=计算机软件1班, students=null)

	DEBUG [main] - ==> Preparing: select id,studentname,gender,age,classno from student where id=?
	DEBUG [main] - ==> Parameters:(Integer)
	DEBUG [main] - ====> Preparing: select cid,cname from classes where cid=?
	DEBUG [main] - ====> Parameters:(String)
	DEBUG [main] - <==== Total:
	DEBUG [main] - <== Total:
	学生编号：,学生姓名：张飞,学生性别：男,学生年龄：18
	所在班级:
	Classes(cid=, cname=计算机软件1班, students=null)

	create table employee (
	empid double ,
	empname varchar (),
	job varchar (),
	leader double
	);
	insert into employee (empid, empname, job, leader) values('','jack','clerk','3');
	insert into employee (empid, empname, job, leader) values('','mike','salesman','3');
	insert into employee (empid, empname, job, leader) values('','john','manager','4');
	insert into employee (empid, empname, job, leader) values('','smith','president',NULL);
	insert into employee (empid, empname, job, leader) values('','rose','salesman','3');

	package cn.kgc.my.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();
	}
	}

	package cn.kgc.my.mapper;

	import cn.kgc.my.entity.Employee;

	public interface EmployeeMapper {
	Employee findEmployeeAndLeaderById(int id);
	}

	DEBUG [main] - ==> Preparing: select * from employee where empid=?
	DEBUG [main] - ==> Parameters:(Integer)
	DEBUG [main] - ====> Preparing: select * from employee where empid=?
	DEBUG [main] - ====> Parameters:(Integer)
	DEBUG [main] - ======> Preparing: select * from employee where empid=?
	DEBUG [main] - ======> Parameters:(Integer)
	DEBUG [main] - <====== Total:
	DEBUG [main] - <==== Total:
	DEBUG [main] - <== Total:
	员工编号:,员工姓名:jack,员工职位:clerk
	他的上司是:员工编号:,员工姓名:john,员工职位:manager

	<!-- 一对多的方式实现自连接 -->
	<resultMap id="empResultMap" 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="empResultMap">
	select * from employee where empid=#{empid}
	</select>

	@Test
	public void testTestFindLeaderAndEmployeesById() {
	SqlSession sqlSession = factory.openSession(true);
	EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);
	Employee leader=mapper.findLeaderAndEmployeesById();
	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());
	}
	}

	private List<Course> courses;
	public List<Course> getCourses() {
	return courses;
	}
	public void setCourses(List<Course> courses) {
	this.courses = courses;
	}

MyBatis多表关联查询的实现示例

目录

一对多查询

多对一查询

自连接查询

多对多查询

	package cn.kgc.my.mapper;

	import cn.kgc.my.entity.Classes;
	import cn.kgc.my.entity.Course;
	import cn.kgc.my.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();
	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());
	}
	}
	}

	package cn.kgc.my.mapper;

	import cn.kgc.my.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);
	}