MyBatis一对一关联查询
在 MyBatis 中，通过 <resultMap> 元素的子元素 <association> 处理一对一级联关系。示例代码如下。
在 <association> 元素中通常使用以下属性。<association property="studentCard" column="cardId"
javaType="net.biancheng.po.StudentCard"
select="net.biancheng.mapper.StudentCardMapper.selectStuCardById" />
property：指定映射到实体类的对象属性。
column：指定表中对应的字段（即查询返回的列名）。
javaType：指定映射到实体对象属性的类型。
select：指定引入嵌套查询的子 SQL 语句，该属性用于关联映射中的嵌套查询。
一对一关联查询可采用以下两种方式：
单步查询，通过关联查询实现
分步查询，通过两次或多次查询，为一对一关系的实体 Bean 赋值
示例
下面以学生和学号为例讲解一对一关联查询的处理过程。1）创建数据表
创建 student（学生）和 studentcard（学号）数据表，SQL 语句如下。
CREATE TABLE `student` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(20) CHARACTER SET utf8 COLLATE utf8_unicode_ci DEFAULT NULL,
`sex` tinyint(4) DEFAULT NULL,
`cardId` int(20) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `cardId` (`cardId`),
CONSTRAINT `student_ibfk_1` FOREIGN KEY (`cardId`) REFERENCES `studentcard` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;
insert into `student`(`id`,`name`,`sex`,`cardId`) values (1,'C语言中文网',0,1),(2,'编程帮',0,2),(3,'赵小红',1,3),(4,'李晓明',0,4),(5,'李紫薇',1,5),(6,'钱百百',0,NULL);
DROP TABLE IF EXISTS `studentcard`;
CREATE TABLE `studentcard` (
`id` int(20) NOT NULL AUTO_INCREMENT,
`studentId` int(20) DEFAULT NULL,
`startDate` date DEFAULT NULL,
`endDate` date DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `studentId` (`studentId`)
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8;
insert into `studentcard`(`id`,`studentId`,`startDate`,`endDate`) values (1,20200311,'2021-03-01','2021-03-11'),(2,20200314,'2021-03-01','2021-03-11'),(3,20200709,'2021-03-01','2021-03-11'),(4,20200508,'2021-03-01','2021-03-11'),(5,20207820,'2021-03-01','2021-03-11');
2）创建持久化类
在 myBatisDemo 应用的 net.biancheng.po 包下创建数据表对应的持久化类 Student 和 StudentCard。Student 的代码如下：
package net.biancheng.po;
public class Student {
private int id;
private String name;
private int sex;
private StudentCard studentCard;
/*省略setter和getter方法*/
@Override
public String toString() {
return "Student [id=" + id + ", name=" + name + ", sex=" + sex + ", studentCard=" + studentCard + "]";
}
}
StudentCard 的代码如下：
package net.biancheng.po;
import java.util.Date;
public class StudentCard {
private int id;
private int studentId;
private Date startDate;
private Date endDate;
/*省略setter和getter方法*/
@Override
public String toString() {
return "StudentCard [id=" + id + ", studentId=" + studentId + "]";
}
}
分步查询
新建 StudentCardMapper 类，代码如下。
package net.biancheng.mapper;
import net.biancheng.po.StudentCard;
public interface StudentCardMapper {
public StudentCard selectStuCardById(int id);
}
StudentCardMapper.xml 对应映射 SQL 语句代码如下。
<mapper namespace="net.biancheng.mapper.StudentCardMapper">
<select id="selectStuCardById"
resultType="net.biancheng.po.StudentCard">
SELECT * FROM studentCard WHERE id = #{id}
</select>
</mapper>
StudentMapper 类方法代码如下。
package net.biancheng.mapper;
import net.biancheng.po.Student;
public interface StudentMapper {
public Student selectStuById1(int id);
public Student selectStuById2(int id);
}
StudentMapper.xml 代码如下。
<mapper namespace="net.biancheng.mapper.StudentMapper">
<!-- 一对一根据id查询学生信息：级联查询的种方法（嵌套查询，执行两个SQL语句） -->
<resultMap type="net.biancheng.po.Student" id="cardAndStu1">
<id property="id" column="id" />
<result property="name" column="name" />
<result property="sex" column="sex" />
<!-- 一对一级联查询 -->
<association property="studentCard" column="cardId"
javaType="net.biancheng.po.StudentCard"
select="net.biancheng.mapper.StudentCardMapper.selectStuCardById" />
</resultMap>
<select id="selectStuById1" parameterType="Integer"
resultMap="cardAndStu1">
select * from student where id=#{id}
</select>
</mapper>
测试代码如下。
public class Test {
public static void main(String[] args) throws IOException {
InputStream config = Resources.getResourceAsStream("mybatis-config.xml");
SqlSessionFactory ssf = new SqlSessionFactoryBuilder().build(config);
SqlSession ss = ssf.openSession();
Student stu = ss.getMapper(StudentMapper.class).selectStuById1(2);
System.out.println(stu);
}
}
运行结果如下。
DEBUG [main] - ==> Preparing: select * from student where id=?
DEBUG [main] - ==> Parameters: 2(Integer)
DEBUG [main] - ====> Preparing: SELECT * FROM studentCard WHERE id = ?
DEBUG [main] - ====> Parameters: 2(Integer)
DEBUG [main] - <==== Total: 1
DEBUG [main] - <== Total: 1
Student [id=2, name=编程帮, sex=0, studentCard=StudentCard [id=2, studentId=20200314]]
单步查询
在 StudentMapper.xml 中添加以下代码。
<resultMap type="net.biancheng.po.Student" id="cardAndStu2">
<id property="id" column="id" />
<result property="name" column="name" />
<result property="sex" column="sex" />
<!-- 一对一级联查询 -->
<association property="studentCard"
javaType="net.biancheng.po.StudentCard">
<id property="id" column="id" />
<result property="studentId" column="studentId" />
</association>
</resultMap>
<select id="selectStuById2" parameterType="Integer"
resultMap="cardAndStu2">
SELECT s.*,sc.studentId FROM student s,studentCard sc
WHERE
s.cardId = sc.id AND s.id=#{id}
</select>
在 StudentMapper 中添加以下方法。
public Student selectStuById2(int id);
运行结果如下。
DEBUG [main] - ==> Preparing: SELECT s.*,sc.studentId FROM student s,studentCard sc WHERE s.cardId = sc.id AND s.id=?
DEBUG [main] - ==> Parameters: 2(Integer)
DEBUG [main] - <== Total: 1
Student [id=2, name=编程帮, sex=0, studentCard=StudentCard [id=2, studentId=20200314]]