0, environment construction
There are two entity sets: student and teacher. Assume that the relationship between the student and the counselor teacher is that one teacher teaches multiple students, and one student has only one counselor teacher. Build tables, build maven projects, import dependencies, write configuration files, etc.
1. Many to one
A student has only one counselor teacher, then the student entity class (pojo, javabean) can be written like this
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;
}
}
The teacher's entity class can be written like this
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;
}
}
Student’s interface, see notes for questions
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();
}
Binding the configuration file of the student interface, the realization of the problem and the points that need attention, see the comments (the usage of the result set mapping is also in the comments). The following code nesting and joining table two methods are implemented
<?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>
Test code
@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. Many to one
The teacher's entity class can be written like this
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;
}
}
The student entity class can be written like this
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;
}
}
Teacher interface
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);
}
Binding the configuration file of the teacher interface, pay attention to the notes, most of which are similar to one-to-many
<?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>
test
@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();
}