假定每一个信用卡客户具有唯一的客户号。如果实体“客户信息”属性依次为“客户号、客户姓名、证件号码、开卡时间、卡状态”,其中,一个客户可以有多个卡,每个卡对应唯一的客户号;实体“客户交易记录”属性依次为“客户号、交易日期、交易开始时间、交易结束时间,交易类型、交易结果、交易金额”,其中一个客户一天可以进行多个交易,但同一时间只能进行一个交易。那么,实体“客户信息”的主键是(40),实体“客户交易记录”的主键是(41),这两个实体型间(42)联系。
(36)
(37)
(34)
(35)
Basically, a(71)is a standalone process or a set of integrated processes that runs on a router or server to control the flow of(72)application traffic(73)through it. Typically, firewalls are(74)on the entry point to a public network such as the Internet. They could be considered traffic cops. The firewall' s(75)is to ensure that all communication between an organization's network and the Internet conform. to the organization's security policies.
(32)
(31)
(30)
(29)
Software design is a(66)process. It requires a certain(67)of flair on the part of the designer. Design can net be learned from a book. It must be practiced and learnt by experience and study of existing systems. A well(68)software system is straight forward to implement and maintain, easily(69)and reliable. Badly(68)software systems, although they may work are(70)to be expensive to maintain, difficult to test and unreliable.
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