[程序员] Mybatis 的 collection 标签可以起到 group by 的作用? - V2EX
V2EX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_idgroup 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?
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