MyBatis多表操作情形一：一对一（方式2：多条sql语句分别查询）

Dear 丶 2022-05-12 02:50 154阅读 0赞

*  与方式一的区别：查询时没有用到左连接，实际是作为两条sql语句进行查询的，在association标签里配置了子查询，而不是直接指定javaType，代码如下：
 *  好处：MyBatis3.0后可以用方式一进行多表查询，但是如方式二这种分多条语句查询，可以做到按需查询，也就是懒加载。多个表一一关联，有时候并不需要查询到底，而是在多个层级间按需自动发出sql指令进行查询，动态代理，可以导入cglib.jar并在settings标签里配置懒加载。  
    Person.xml

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper PUBLIC "-//ibatis.apache.org//DTD SQL Map 3.0//EN"
    "http://ibatis.apache.org/dtd/ibatis-3-mapper.dtd">
    <mapper namespace="com.yicha.entity.Person">
    	
    	<resultMap type="Person" id="myMap">
    		<result property="name" column="name"/>
    		<result property="age" column="age"/>
    		
    		
    		<association property="card" javaType="Card">
    			<result property="cardNum" column="cardnum"/>
    		</association>
    	</resultMap>
    	
    	<select id="findPerson" resultMap="myMap">
    		SELECT a.name,a.age,b.cardnum 
    		FROM person a
    		LEFT JOIN card b
    		ON a.cardid=b.id
    		WHERE a.id=1
    	</select>
    	
    	
    	<resultMap type="Person" id="myMap2">
    		<result property="name" column="name"/>
    		<result property="age" column="age"/>
    		<association property="card" column="cardid" select="com.yicha.entity.Card.find2"></association>
    	</resultMap>
    	
    	
    	<select id="find1" resultMap="myMap2">
    		SELECT name,age,cardid
    		FROM person
    		WHERE id=1
    	</select>
    </mapper>

Card.xml

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE mapper PUBLIC "-//ibatis.apache.org//DTD SQL Map 3.0//EN"
    "http://ibatis.apache.org/dtd/ibatis-3-mapper.dtd">
    <mapper namespace="com.yicha.entity.Card">
    	<select id="find2" resultType="Card">
    		SELECT cardnum
    		FROM card
    		WHERE id=#{cardid}
    	</select>
    </mapper>

SqlMapConfig.xml  
![在这里插入图片描述][70]

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE configuration PUBLIC "-//ibatis.apache.org//DTD SQL Map Config 3.0//EN"
    "http://ibatis.apache.org/dtd/ibatis-3-config.dtd">
    <configuration>
    	
    	<typeAliases>
    		<typeAlias type="com.yicha.entity.Person" alias="Person"/>
    		<typeAlias type="com.yicha.entity.Card" alias="Card"/>
    	</typeAliases>
    	
    	
    	<environments default="e1">
    		<environment id="e1">
    			
    			<transactionManager type="JDBC"></transactionManager>
    			
    			<dataSource type="POOLED">
    				<property name="driver" value="com.mysql.jdbc.Driver"/>
    				<property name="url" value="jdbc:mysql://localhost:3306/test"/>
    				<property name="username" value="root"/>
    				<property name="password" value="root1205"/>
    			</dataSource>
    		</environment>
    	</environments>
    	
    	
    	<mappers>
    		<mapper resource="com/yicha/entity/Person.xml"/>
    		<mapper resource="com/yicha/entity/Card.xml"/>
    	</mappers>
    </configuration>

结果：  
![这里写图片描述][70 1]

[70]: /images/20220512/f1f4723bef8747b5b6525a73970add38.png
[70 1]: /images/20220512/aacd21d9cc5d410d8b8cb9ef67d8a130.png