*1,创建表和数据*
创建一张教师表和一张班级表,一个老师负责一个班级,那么他们之间的关系就是一对一的关系。
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('teacher1');
INSERT INTO teacher(t_name) VALUES('teacher2');
INSERT INTO class(c_name, teacher_id) VALUES('class_a', 1);
INSERT INTO class(c_name, teacher_id) VALUES('class_b', 2);
表之间的关系就是:
2,定义实体类
package me.gacl.domain;
/**
* @author gacl
* 定义teacher表对应的实体类
*/
public class Teacher {
//定义实体类的属性,与teacher表中的字段对应
private int id; //id===>t_id
private String name; //name===>t_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 + "]";
}
}
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;
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 +"]";
}
}
3,定义sql映射文件classMapper.xml
<?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">
<!-- 为这个mapper指定一个唯一的namespace,namespace的值习惯上设置成包名+sql映射文件名,这样就能够保证namespace的值是唯一的
例如namespace="me.gacl.mapping.classMapper"就是me.gacl.mapping(包名)+classMapper(classMapper.xml文件去除后缀)
-->
<mapper namespace="me.gacl.mapping.classMapper">
<!--
根据班级id查询班级信息(带老师的信息)
##1. 联表查询
SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=1;
##2. 执行两次查询
SELECT * FROM class WHERE c_id=1; //teacher_id=1
SELECT * FROM teacher WHERE t_id=1;//使用上面得到的teacher_id--> <!-- 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 封装联表查询的数据(去除重复的数据) select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=1 -->
<select id="getClass" parameterType="int" resultMap="ClassResultMap">
select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
</select>
<!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->
<resultMap type="me.gacl.domain.Classes" id="ClassResultMap">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" javaType="me.gacl.domain.Teacher">
<id property="id" column="t_id"/>
<result property="name" column="t_name"/>
</association>
</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 id="getClass2" parameterType="int" resultMap="ClassResultMap2">
select * from class where c_id=#{id}
</select>
<!-- 使用resultMap映射实体类和字段之间的一一对应关系 -->
<resultMap type="me.gacl.domain.Classes" id="ClassResultMap2">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" column="teacher_id" select="getTeacher"/>
</resultMap>
<select id="getTeacher" parameterType="int" resultType="me.gacl.domain.Teacher">
SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
</select>
</mapper>
在conf.xml文件中注册classMapper.xml
<mappers>
<mapper resource="me/gacl/mapping/classMapper.xml"/>
</mappers>
4,测试类代码
package me.gacl.test;
import me.gacl.domain.Classes;
import me.gacl.test.MyBatisUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
public class Test3 {
@Test
public void testGetClass(){
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,
* getClass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.getClass";//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();
System.out.println(clazz);//打印结果:Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1]]
}
@Test
public void testGetClass2(){
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,
* getClass2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.getClass2";//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();
System.out.println(clazz);//打印结果:Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1]]
}
}
总结
MyBatis中使用association标签来解决一对一的关联查询,association标签可用的属性如下:
• property:对象属性的名称
• javaType:对象属性的类型
• column:所对应的外键字段名称
• select:使用另一个查询封装的结果