有一个 java 类；

@Data @Alias("Customer") public class Customer {     private String custName;     private String address;     private List<Order> orderList; } 

    <select id="getCustomer" resultMap="CustomerAndOrder">         select * from test.orders a, test.customers b where a.cust_id = b.cust_id     </select>          <resultMap id="CustomerAndOrder" type="Customer">         <result property="custName" column="cust_name"/>         <result property="address" column="cust_address"/>         <collection property="orderList" ofType="Order">             <result property="orderNum" column="order_num"/>             <result property="orderDate" column="order_date"/>         </collection>     </resultMap> 

getCustomer 方法是为了查询每个顾客，都有哪些订单。

执行 getCustomer 方法后，可以看到这个链表查询起到了类似 group by 的作用，以 cust_name 和 cust_address 分组，将每个组内的订单信息弄进了一个集合内。类似于：

select  cust_name,  cust_address, (select 组内各行的 order_num,order_date into a List) from test.orders a, test.customers b where a.cust_id = b.cust_id group by cust_name, cust_address 

大概是上面这个意思，但实际上 mysql 无法做到这种语句。

然后我想问的是：

• 假设(select 组内各行的 order_num,order_date into a List)这种行为支持的话，上面这种查询是否等价于上面这 sql 语句？
• 如果等价，出现在<result property=”xxx” column=”yyy”/>的 yyy 一定是 group by 的条件呗？

另外，mybatis 官方文档的这个例子：

<!-- 非常复杂的结果映射 --> <resultMap id="detailedBlogResultMap" type="Blog">  <constructor>    <idArg column="blog_id" javaType="int"/>  </constructor>  <result property="title" column="blog_title"/>  <association property="author" javaType="Author">    <id property="id" column="author_id"/>    <result property="username" column="author_username"/>    <result property="password" column="author_password"/>    <result property="email" column="author_email"/>    <result property="bio" column="author_bio"/>    <result property="favouriteSection" column="author_favourite_section"/>  </association>  <collection property="posts" ofType="Post">    <id property="id" column="post_id"/>    <result property="subject" column="post_subject"/>    <association property="author" javaType="Author"/>    <collection property="comments" ofType="Comment">      <id property="id" column="comment_id"/>    </collection>    <collection property="tags" ofType="Tag" >      <id property="id" column="tag_id"/>    </collection>    <discriminator javaType="int" column="draft">      <case value="1" resultType="DraftPost"/>    </discriminator>  </collection> </resultMap> 

是否可以认为其等价 sql 语句是 group by blog_title, author_id, author_username, author_password, author_email, author_bio, author_favourite_section?