Mybatis一对一关联查询
有两张表，老师表teacher和班级表class，一个class班级对应一个teacher，一个teacher对应一个class
需求是根据班级id查询班级信息(带老师的信息)
创建teacher和class表：
CREATE TABLE teacher (
t_id INT PRIMARY KEY AUTO_INCREMENT,
t_name VARCHAR(20)
);
CREATE TABLE class (
c_id INT PRIMARY KEY AUTO_INCREMENT,
c_name VARCHAR(20),
teacher_id INT
);
ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES
teacher(t_id);
INSERT INTO teacher(t_name) VALUES(‘LS1‘);
INSERT INTO teacher(t_name) VALUES(‘LS2‘);
INSERT INTO class(c_name, teacher_id) VALUES(‘bj_a‘, 1);
INSERT INTO class(c_name, teacher_id) VALUES(‘bj_b‘, 2);
首先定义实体类
老师Teacher类
package mybatis.bean;
public class Teacher {
private int id;
private String name;
public Teacher() {
}
public Teacher(int id, String name) {
super();
this.id = id;
this.name = 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;
}
@Override
public String toString() {
return "Teacher [id=" + id + ", name=" + name + "]";
}
}
班级Class类
package mybatis.bean;
public class Class {
private int id;
private String name;
private Teacher teacher;
public Class() {
}
public Class(int id, String name, Teacher teacher) {
super();
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;
}
@Override
public String toString() {
return "Class [id=" + id + ", name=" + name + ", teacher=" + teacher + "]";
}
}
要实现一对一关联查询，有两种方式
1.关联表查询
SELECT * FROM class c, teacher t WHERE c.teacher_id = t.t_id AND c.c_id = 1;
2.执行2次查询
先查询class表，获取teacher_id，再查询teacher表，获取teacher的信息
SELECT * FROM class where c_id = 1; SELECT * FROM teacher where t_id = 1;//使用上面得到的teacher_id
这两种查询方式就引出了2中方式
- 嵌套结果: 使用嵌套结果映射来处理重复的联合结果的子集
- 嵌套查询：通过执行另外一个SQL 映射语句来返回预期的复杂类型
二、一对多关联
创建表和数据
在上面的一对一关联查询演示中，我们已经创建了班级表和教师表，因此这里再创建一张学生表
CREATE TABLE student(
s_id INT PRIMARY KEY AUTO_INCREMENT,
s_name VARCHAR(20),
class_id INT
);
INSERT INTO student(s_name, class_id) VALUES(‘student_A‘, 1);
INSERT INTO student(s_name, class_id) VALUES(‘student_B‘, 1);
INSERT INTO student(s_name, class_id) VALUES(‘student_C‘, 1);
INSERT INTO student(s_name, class_id) VALUES(‘student_D‘, 2);
INSERT INTO student(s_name, class_id) VALUES(‘student_E‘, 2);
INSERT INTO student(s_name, class_id) VALUES(‘student_F‘, 2);
定义实体类
1、Student类
package me.gacl.domain;
/**
* @author gacl
* 定义student表所对应的实体类
*/
public class Student {
//定义属性，和student表中的字段对应
private int id; //id===>s_id
private String name; //name===>s_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;
}
@Override
public String toString() {
return "Student [id=" + id + ", name=" + name + "]";
}
}
2、修改Classes类，添加一个List<Student> students属性，使用一个List<Student>集合属性表示班级拥有的学生，如下：
package me.gacl.domain;
import java.util.List;
/**
* @author gacl
* 定义class表对应的实体类
*/
public class Classes {
//定义实体类的属性，与class表中的字段对应
private int id; //id===>c_id
private String name; //name===>c_name
/**
* class表中有一个teacher_id字段，所以在Classes类中定义一个teacher属性，
* 用于维护teacher和class之间的一对一关系，通过这个teacher属性就可以知道这个班级是由哪个老师负责的
*/
private Teacher teacher;
//使用一个List<Student>集合属性表示班级拥有的学生
private List<Student> students;
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;
}
public List<Student> getStudents() {
return students;
}
public void setStudents(List<Student> students) {
this.students = students;
}
@Override
public String toString() {
return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher
+ ", students=" + students + "]";
}
}
修改sql映射文件classMapper.xml
添加如下的SQL映射信息
<!--
根据classId查询对应的班级信息,包括学生,老师
-->
<!--
方式一: 嵌套结果: 使用嵌套结果映射来处理重复的联合结果的子集
SELECT * FROM class c, teacher t,student s WHERE c.teacher_id=t.t_id AND c.C_id=s.class_id AND c.c_id=1
-->
<select id="getClass3" parameterType="int" resultMap="ClassResultMap3">
select * from class c, teacher t,student s where c.teacher_id=t.t_id and c.C_id=s.class_id and c.c_id=#{id}
</select>
<resultMap type="me.gacl.domain.Classes" id="ClassResultMap3">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher">
<id property="id" column="t_id"/>
<result property="name" column="t_name"/>
</association>
<!-- ofType指定students集合中的对象类型 -->
<collection property="students" ofType="me.gacl.domain.Student">
<id property="id" column="s_id"/>
<result property="name" column="s_name"/>
</collection>
</resultMap>
<!--
方式二：嵌套查询：通过执行另外一个SQL映射语句来返回预期的复杂类型
SELECT * FROM class WHERE c_id=1;
SELECT * FROM teacher WHERE t_id=1 //1 是上一个查询得到的teacher_id的值
SELECT * FROM student WHERE class_id=1 //1是第一个查询得到的c_id字段的值
-->
<select id="getClass4" parameterType="int" resultMap="ClassResultMap4">
select * from class where c_id=#{id}
</select>
<resultMap type="me.gacl.domain.Classes" id="ClassResultMap4">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher" select="getTeacher2"></association>
<collection property="students" ofType="me.gacl.domain.Student" column="c_id" select="getStudent"></collection>
</resultMap>
<select id="getTeacher2" parameterType="int" resultType="me.gacl.domain.Teacher">
SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
</select>
<select id="getStudent" parameterType="int" resultType="me.gacl.domain.Student">
SELECT s_id id, s_name name FROM student WHERE class_id=#{id}
</select>
编写单元测试代码
package me.gacl.test;
import me.gacl.domain.Classes;
import me.gacl.util.MyBatisUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
public class Test4 {
@Test
public void testGetClass3(){
SqlSession sqlSession = MyBatisUtil.getSqlSession();
/**
* 映射sql的标识字符串，
* me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，
* getClass3是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL
*/
String statement = "me.gacl.mapping.classMapper.getClass3";//映射sql的标识字符串
//执行查询操作，将查询结果自动封装成Classes对象返回
Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录
//使用SqlSession执行完SQL之后需要关闭SqlSession
sqlSession.close();
//打印结果：Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1], students=[Student [id=1, name=student_A], Student [id=2, name=student_B], Student [id=3, name=student_C]]]
System.out.println(clazz);
}
@Test
public void testGetClass4(){
SqlSession sqlSession = MyBatisUtil.getSqlSession();
/**
* 映射sql的标识字符串，
* me.gacl.mapping.classMapper是classMapper.xml文件中mapper标签的namespace属性的值，
* getClass4是select标签的id属性值，通过select标签的id属性值就可以找到要执行的SQL
*/
String statement = "me.gacl.mapping.classMapper.getClass4";//映射sql的标识字符串
//执行查询操作，将查询结果自动封装成Classes对象返回
Classes clazz = sqlSession.selectOne(statement,1);//查询class表中id为1的记录
//使用SqlSession执行完SQL之后需要关闭SqlSession
sqlSession.close();
//打印结果：Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1], students=[Student [id=1, name=student_A], Student [id=2, name=student_B], Student [id=3, name=student_C]]]
System.out.println(clazz);
}
}
原文地址：https://www.cnblogs.com/wuqi2328857945/p/9621760.html
时间： 2024-08-05 16:12:55