<association>关联的结果查询

association：用于对象间包含关系映射

方式一：通过association标签来封装结果集

<mapper namespace="org.apache.ibatis.submitted.associationtest.Mapper">  
    <resultMap type="org.apache.ibatis.submitted.associationtest.Car" id="carResult">  
        <id column="carid" property="id"/>  
        <result column="cartype" property="type"/>  
        <association property="engine" resultMap="engineResult"/>  
        <association property="brakes" resultMap="brakesResult"/>  
    </resultMap>  
    <resultMap type="org.apache.ibatis.submitted.associationtest.Engine" id="engineResult">  
        <result column="enginetype" property="type"/>  
        <result column="enginecylinders" property="cylinders"/>  
    </resultMap>  
    <resultMap type="org.apache.ibatis.submitted.associationtest.Brakes" id="brakesResult">  
        <result column="brakesType" property="type"/>  
    </resultMap>  
    <select id="getCars" resultMap="carResult">  
        select * from cars  
    </select>  
    <select id="getCarsNonUnique" resultMap="carResult">  
        select 1 as carid, cartype, enginetype, enginecylinders, brakestype from cars  
    </select>  
    <select id="getCars2" resultMap="carResult">  
        select 1 as carid, cartype, enginetype, enginecylinders, brakestype from cars where carid in (1,2)  
    </select>  
</mapper>  

查询：

<select id="getEmpAndDept" resultMap="complexEmp2">
    SELECT e.id id,e.last_name last_name,e.email email,e.gender gender,e.d_id d_id,d.id did,d.dept_name dept_name FROM tb1_emplyee e,tb1_dept d WHERE e.d_id=d.id AND e.id=#{id}
</select>

方式二：通过association标签实现分段查找

<resultMap id="MyEmpByStep" type="com.test.beans.Employee">
    <id column="id" property="id"/>
    <result column="last_name" property="lastName"/>
    <result column="email" property="email"/>
    <result column="gender" property="gender"/>
    <!-- association定义关联对象的封装规则
        select:表明当前属性是调用select指定的方法查出的结果
        column:指定将哪一列的值传给这个方法
        流程：使用select指定的方法（传入column指定的这列参数的值）查出对象，并封装给property指定的属性
    -->
    <association property="dept" select="com.test.dao.DepartMentMapper.getDeptById" column="d_id"> </association>
</resultMap>

 对应的Department映射文件:

<mapper namespace="com.test.dao.DepartMentMapper">
    <select id="getDeptById" resultType="com.test.beans.Department">
        SELECT id,dept_name departmentName FROM tb1_dept WHERE id=#{id}
    </select>
</mapper>

查询：

<select id="getEmpByIdStep" resultMap="MyEmpByStep">
    select * from tb1_emplyee where id=#{id}
</select>

association标签还可以实现懒加载的功能

什么是懒加载呢?

前面的分步查询,每查询一次都会执行两次sql(一次查询员工,一次查询部门)

有时候我们并不需要插叙部门的情况,所以懒查询就可以帮我们解决这个问题,提高效率,减少资源的占用

懒加载的实现也非常简单,只要在全局配置文件中添加如下的配置代码即可:

<settings>
    <!--显示的指定每个我们需要更改的配置的值，即使他是默认的。防止版本更新带来的问题  -->
    <setting name="lazyLoadingEnabled" value="true"/>
    <setting name="aggressiveLazyLoading" value="false"/>
</settings>