Question

我正在尝试实施一个图书馆管理数据库，我想根据贷款日期推断一个返回日期，loandays，它将保留我们允许用户拥有该书的天数。到目前为止，这是我的代码：

CREATE TABLE LOAN 
( 
    COPY_ID int CONSTRAINT LOAN_COPY_FK 
            FOREIGN KEY(COPY_ID) REFERENCES COPY(COPY_ID) NOT NULL, 
    BORROWER_ID int CONSTRAINT LOAN_BORROWER_FK 
            FOREIGN KEY (BORROWER_ID) REFERENCES BORROWER (BORROWER_ID) NOT NULL, 
    LOAN_DATE DATE NOT NULL CONSTRAINT check_loan_date 
            CHECK (LOAN_DATE < RETURN_DATE),
    LOAN_DAYS int NOT NULL, 
    RETURN_DATE AS (LOAN_DATE + LOAN_DAYS) CONSTRAINT LOAN_PK 
            PRIMARY KEY(COPY_ID, BORROWER_ID) 
)

我有一个错误，因为LOAN_DAYS是整数，我不知道如何把它当作没有月份和年份的日期。

Answer 1

您应该计算RETURN_DATE AS DATEADD(day, LOAN_DAYS, LOAN_DATE)，并且不能在检查约束RETURN_DATE中使用计算出的CONSTRAINT check_loan_date CHECK (LOAN_DATE < RETURN_DATE)。

相反，我建议使用LOAD_DAYS大于0的检查约束，这可以保证RETURN_DATE大于LOAD_DATE，如下所示：

CREATE TABLE LOAN 
( 
    COPY_ID int CONSTRAINT LOAN_COPY_FK FOREIGN KEY(COPY_ID) REFERENCES COPY(COPY_ID) NOT NULL, 
    BORROWER_ID int CONSTRAINT LOAN_BORROWER_FK FOREIGN KEY (BORROWER_ID) REFERENCES BORROWER (BORROWER_ID) NOT NULL, 
    LOAN_DATE DATE NOT NULL ,
    LOAN_DAYS int NOT NULL CONSTRAINT check_loan_days CHECK (LOAN_DAYS > 0), 
    RETURN_DATE AS DATEADD(day,LOAN_DAYS,LOAN_DATE) CONSTRAINT LOAN_PK PRIMARY KEY(COPY_ID, BORROWER_ID) 
)

计算贷款日期的退货日期

1 个答案: