0、环境搭建
有学生和老师两个实体集，假设学生与辅导员老师的关系是一位老师教多个学生，一个学生只有一位辅导员老师。建表、建maven项目、导入依赖、写配置文件等等
1、多对一
一个学生只有一位辅导员老师，则学生的实体类(pojo、javabean)可以这样写
package com.wt.pojo;
public class Student {
private int id;
private String name;
private Teacher teacher;
public Student() {
}
public Student(int id, String name, Teacher teacher) {
this.id = id;
this.name = name;
this.teacher = teacher;
}
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 Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
}
老师的实体类可以这样写
package com.wt.pojo;
public class Teacher {
private int tid;
private String name;
public Teacher(int tid, String name) {
this.tid = tid;
this.name = name;
}
public Teacher() {
}
public int getTid() {
return tid;
}
public void setTid(int tid) {
this.tid = tid;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
学生的接口，问题看注释
package com.wt.mapper;
import com.wt.pojo.Student;
import java.util.List;
public interface StudentMapper {
//想要实现这样的select语句:
//select student.id,student.name,teacher.name
//from student,teacher
//where student.tid=teacher.id
//如果直接执行是查不出的，必须再=在配置文件中“模拟”联表查询或嵌套查询
//获得所有学生，模拟嵌套查询
List<Student> getAllStudents01();
//获得所有学生，模拟联表查询
List<Student> getAllStudents02();
}
绑定学生接口的配置文件，问题的实现以及需要注意的点看注释（结果集映射的用法也在注释里）。下面的代码嵌套和联表两种方法都实现了
<?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.wt.mapper.StudentMapper">
<!--resultMap是结果集映射，这个需要在下面再写一个resultMap标签来完成结果集映射，当然resultMap的id要对好-->
<select id="getAllStudents01" resultMap="StudentTeacher01">
select * from mybatis.student;
</select>
<!--下面的结果集映射将Student中的属性(property)与表中的列(column)相映射起来-->
<resultMap id="StudentTeacher01" type="Student">
<result property="id" column="id"/>
<result property="name" column="name"/>
<!--对于简单的属性，直接写映射关系就好了，甚至不写也可以，mybatis会自动帮你对应(有前提)-->
<!--对于复杂的属性，比如引用，就得用association或者collection了，用法参照下面-->
<!--Student中的teacher属性是个引用，想要查询到，就得嵌套其他的查询-->
<association property="teacher" column="tid" javaType="Teacher" select="getTeacherById"/>
</resultMap>
<!--下面这个查询标签的id对应第16行中的select的值，所以这个查询会被上面“调用”，参数也会自动传递(比如这里的tid)-->
<select id="getTeacherById" resultType="Teacher">
select * from mybatis.teacher where id = #{tid};
</select>
<!--以上的三个标签实现了嵌套查询-->
<!--=====================================另一种方式(类似联表查询)=====================================-->
<select id="getAllStudents02" resultMap="StudentTeacher02">
select student.id sid,student.name sname,teacher.name tname
from student,teacher
where student.tid=teacher.id
</select>
<resultMap id="StudentTeacher02" type="Student">
<result property="id" column="sid"/>
<result property="name" column="sname"/>
<association property="teacher" javaType="Teacher">
<result property="name" column="tname"/>
</association>
</resultMap>
</mapper>
测试的代码
@org.junit.Test
public void test00(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
List<Student> students = studentMapper.getAllStudents01();
for(Student student:students){
System.out.print(student.getId());
System.out.print(" ");
System.out.println(student.getTeacher().getName());
}
System.out.println("--------------------------------");
List<Student> students2 = studentMapper.getAllStudents02();
for(Student student:students2){
System.out.print(student.getId());
System.out.print(" ");
System.out.println(student.getTeacher().getName());
}
sqlSession.close();
}
2、多对一
老师的实体类可以这样写
package com.wt.pojo;
import java.util.List;
public class Teacher {
private int tid;
private String name;
private List<Student> students;
public Teacher() {
}
public Teacher(int tid, String name, List<Student> students) {
this.tid = tid;
this.name = name;
this.students = students;
}
public int getTid() {
return tid;
}
public void setTid(int tid) {
this.tid = tid;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Student> getStudents() {
return students;
}
public void setStudents(List<Student> students) {
this.students = students;
}
}
学生的实体类可以这样写
package com.wt.pojo;
public class Student {
private int id;
private String name;
private int tid;
public Student() {
}
public Student(int id, String name, int tid) {
this.id = id;
this.name = name;
this.tid = tid;
}
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 int getTid() {
return tid;
}
public void setTid(int tid) {
this.tid = tid;
}
}
老师的接口
package com.wt.mapper;
import com.wt.pojo.Student;
import com.wt.pojo.Teacher;
import java.util.List;
public interface TeacherMapper {
//需求：根据老师的id查询出其所教的所有学生
List<Student> getStudentsOfTeacherByTeacherId();
//列出学生的同时列出老师
Teacher getTeacherAndHerStudents(int tid);
}
绑定老师接口的配置文件，注意看注释，大部分与一对多相似
<?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.wt.mapper.TeacherMapper">
<select id="getStudentsOfTeacherByTeacherId" resultMap="TeacherStudent">
select student.id sid,student.name sname from student,teacher where student.tid=teacher.id and teacher.id=1;
</select>
<resultMap id="TeacherStudent" type="Student">
<result property="name" column="sname"/>
<result property="id" column="sid"/>
</resultMap>
<select id="getTeacherAndHerStudents" resultMap="TeacherStudent2">
select student.id sid,student.name sname,teacher.name tname
from student,teacher
where student.tid=teacher.id and teacher.id=#{tid}
</select>
<resultMap id="TeacherStudent2" type="Teacher">
<result property="name" column="tname"/>
<!--老师拥有一个列表的学生，用collection来表示这个属性，collection中的Type不是javaType而是ofType-->
<collection property="students" ofType="Student">
<result property="name" column="sname"/>
<result property="id" column="sid"/>
</collection>
</resultMap>
</mapper>
测试
@org.junit.Test
public void test01(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
TeacherMapper teacherMapper = sqlSession.getMapper(TeacherMapper.class);
List<Student> students = teacherMapper.getStudentsOfTeacherByTeacherId();
for(Student student:students){
System.out.println(student.getName());
}
sqlSession.close();
}
@org.junit.Test
public void test02(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
TeacherMapper teacherMapper = sqlSession.getMapper(TeacherMapper.class);
Teacher teacher = teacherMapper.getTeacherAndHerStudents(1);
for(Student student:teacher.getStudents()){
System.out.print(student.getId());
System.out.print(student.getName());
System.out.println(teacher.getName());
System.out.println("--------------------------------");
}
sqlSession.close();
}