Java中ResultMap的嵌套查询映射

在Java中，如果需要进行嵌套查询映射，可以使用ResultMap来实现。以下是一个示例代码：

<resultMap id="orderResultMap" type="Order">     <id property="id" column="order_id"/>     <result property="orderDate" column="order_date"/>     <result property="totalAmount" column="total_amount"/>     <association property="customer" column="customer_id" javaType="Customer" select="selectCustomerById"/> </resultMap>  <resultMap id="customerResultMap" type="Customer">     <id property="id" column="customer_id"/>     <result property="name" column="customer_name"/>     <result property="email" column="customer_email"/> </resultMap>  <select id="selectOrderWithCustomer" resultMap="orderResultMap">     SELECT o.id as order_id, o.order_date, o.total_amount, o.customer_id, c.id as customer_id, c.name as customer_name, c.email as customer_email     FROM orders o     JOIN customers c ON o.customer_id = c.id     WHERE o.id = #{orderId} </select>  <select id="selectCustomerById" resultMap="customerResultMap">     SELECT id as customer_id, name as customer_name, email as customer_email     FROM customers     WHERE id = #{customerId} </select> 

在上面的示例中，我们定义了两个ResultMap，分别用于映射Order和Customer对象。在Order对象的ResultMap中，我们使用了association标签来指定在查询Order对象时需要查询关联的Customer对象，并通过select属性指定了selectCustomerById查询语句的映射。在selectOrderWithCustomer查询语句中使用了orderResultMap来映射查询结果，从而实现了嵌套查询映射。