MyBatis的一对一查询实现方式

Mybatis对查询结果的映射有两种, 一种是resultMap, 还有一种是resultType.

reusltType模式实现的映射需要保证数据表中的列名和映射对象的属性名要一致.

resultMap则可以由自己灵活配置.

要实现一对一的查询,先建立两个表, 一个stu表, 和一个card表, 为了简单就少列出几个字段.

-- stu表
CREATE TABLE `f_stu` (
`id` int(11) NOT NULL AUTO_INCREMENT COMMENT '主键',
`name` varchar(10) NOT NULL COMMENT '姓名',
`study_num` varchar(20) NOT NULL COMMENT '学号',
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8

-- card表
CREATE TABLE `f_idcards` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`card_num` varchar(18) DEFAULT NULL COMMENT '卡号',
`u_id` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `u_id` (`u_id`),
CONSTRAINT `f_idcards_ibfk_1` FOREIGN KEY (`u_id`) REFERENCES `f_stu` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8

然后, 引入mybatis需要的jar包, 编写主配置文件mappingConfig.xml:

这里并未使用spring,所以事务交给他本身管理.

<!-- mappingConfig.xml -->
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD SQL Map Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<properties resource="jdbc.properties"></properties>
<typeAliases>
<package name="com.mybatis.po"/>
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC" />
<dataSource type="POOLED">
<property name="driver" value="${jdbc.driverClassName}" />
<property name="url" value="${jdbc.url}" />
<property name="username" value="${jdbc.username}" />
<property name="password" value="${jdbc.password}" />
</dataSource>
</environment>
</environments>
<!-- 加载配置文件 -->
<mappers>
<mapper resource="mapper/StuMapper.xml"/>
</mappers>
</configuration>

接下来,编写对应的StuMapper.xml文件:

<?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.mybatis.dao.StuMapper">

<!-- 查询学生信息和卡号 -->
<select id="findStuAndCard" resultType="stuCardResult">
SELECT A.*, B.card_num
FROM f_IDcards B, f_stu	A
WHERE A.id = B.u_id
</select>
</mapper>

resultType的结果是自定义的pojo:

将查询出来没有的列名映射过去;

public class StuCardResult extends Stu{
private String card_num;

public String getCard_num() {
return card_num;
}

public void setCard_num(String card_num) {
this.card_num = card_num;
}
}

接下来,编写Mapper代理接口:

public interface StuMapper {

public List<StuCardResult> findStuAndCard() throws Exception;

}

由于还没有使用spring,所以在测试的时候为了方便, 编写一个工具类用来获取sqlSession:

public class GetSqlSessionUtil {

private static SqlSessionFactory sqlSessionFactory;

public static SqlSession getSqlSession() throws Exception {
if(sqlSessionFactory == null) {
String resource = "mappingConfig.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
sqlSessionFactory =  new SqlSessionFactoryBuilder().build(inputStream);
}
return sqlSessionFactory.openSession();
}
}

最后,可以来测试一下resultType的方法:

public class StuTest {
@Test
public void test() throws Exception {
SqlSession sqlSession = GetSqlSessionUtil.getSqlSession();
StuMapper stuMapper  = sqlSession.getMapper(StuMapper.class);
List<StuCardResult> list = stuMapper.findStuAndCard();
if(list != null) {
for(StuCardResult result : list) {
System.out.println(result.getCard_num());
}
}
}
}

运行测试单元:


结果正确.

再使用resultMap来进行查询结果映射:

修改配置文件:

<!-- StuMapper.xml -->
<!-- 定义resultMap -->
<resultMap type="stu" id="stuAndCardResultMap">
<id column="id" property="id"/>
<result column="name" property="name"/>
<result column="study_num" property="studyNum"/>
<association property="card" column="u_id" javaType="Card">
<result column="card_num" property="cardNum"/>
</association>
</resultMap>

<select id="findStuAndCardResultMap" resultMap="stuAndCardResultMap">
SELECT A.*, B.card_num
FROM f_IDcards B, f_stu	A
WHERE A.id = B.u_id
</select>

然后修改一下Stu.java类,添加一个card属性进去.

private String studyNum;   //与property映射过来的属性名称一致
private Card card;

mapper代理类中添加响应的接口,然后进行测试:



这样就Ok了.