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12 完整性题 设某电子商务公司的数据库中有下列四张基本表: Customers(Cno, Cname, Csex, Cage, Caddress, Mphone, Email) Goods(Gno, Gname, Gtype, Price, Manufac) Sells(Sno, Sdate, Saddress, Cno, IsPay) Detail(Sno, Gno, Quantity) 客户表属性分别为:客户编号,客户姓名,客户性别,客户年龄,客户地址,手机号,电子邮箱; 商品表属性分别为:商品编号,商品名称,商品类别,价格,生产商; 销售属性分别为:销售,销售日期,送货地址,客户编号,是否已付款; 销售明细表属性分别为:销售,商品编号,数量。 其中各表中各属性的含义如下,且假定在客户付款前 IsPay 取值为 'N' ,付款后 IsPay 取值为 'Y' 。 1 )写出各表的候选码、主码。 2 )写出需要建立的外码。 3 )写出你认为需要设置的用户自定义约束。
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