# 9.1.1\. 每个类分层结构一张表(Table per class hierarchy) 假设我们有接口`Payment`和它的几个实现类： `CreditCardPayment`, `CashPayment`, 和`ChequePayment`。则“每个类分层结构一张表”(Table per class hierarchy)的映射代码如下所示： ``` <class name="Payment" table="PAYMENT"> <id name="id" type="long" column="PAYMENT_ID"> <generator class="native"/> </id> <discriminator column="PAYMENT_TYPE" type="string"/> <property name="amount" column="AMOUNT"/> ... <subclass name="CreditCardPayment" discriminator-value="CREDIT"> <property name="creditCardType" column="CCTYPE"/> ... </subclass> <subclass name="CashPayment" discriminator-value="CASH"> ... </subclass> <subclass name="ChequePayment" discriminator-value="CHEQUE"> ... </subclass> </class> ``` 采用这种策略只需要一张表即可。它有一个很大的限制：要求那些由子类定义的字段， 如`CCTYPE`，不能有`非空(NOT NULL)`约束。