# 9.1.1\. 每個類分層結構一張表(Table per class hierarchy) 假設我們有接口`Payment`和它的幾個實現類： `CreditCardPayment`, `CashPayment`, 和`ChequePayment`。則“每個類分層結構一張表”(Table per class hierarchy)的映射代碼如下所示： ``` <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> ``` 采用這種策略只需要一張表即可。它有一個很大的限制：要求那些由子類定義的字段， 如`CCTYPE`，不能有`非空(NOT NULL)`約束。