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数据库设汁可分为六个阶段，每个阶段都有自己的设计内容，“为哪些关系，在哪些属性上建立什么样的索引”这一设计内容属于数据库设计的哪个阶段
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A granted patent is the result of a bargain struck between an inventor and the state, by which the inventor gets a limited period of monopoly and publishes full details of his invention to the public after the period terminates.Only in the most exceptional circumstances is the life-span of a patent extended to alter this normal process of events.The longest extension ever granted was to George Valensi. His 1939 patent for color TV receiver circuitry was extended until 1971 because for most of the patent’s normal life there was no hope of reward for’ the invention.Because a patent remains permanently public after it has terminated, the shelves of the library attached to the patent office contain details of literally millions of ideas that are free for anyone to use and, if older than half a century, sometimes even re-patent. Indeed, patent experts often advise anyone wishing to avoid the high cost of conducting a search through live patents that the one sure way of avoiding violation of any other inventor’s right is to plagiarize a dead patent. Likewise, because publication of an idea in any other form permanently invalidates further patents on that idea, it is traditionally safe to take ideas from other areas of print. Much modern technological advance is based on these presumptions of legal security.Anyone closely involved in patents and inventions soon learns that most "new" ideas are, in fact, as old as the hills. It is their reduction to commercial practice, either through necessity or dedication, or through the availability of new technology, that makes news and money. The basic patent for the theory of magnetic recording dates back to 1886. Many of the original ideas behind television originate from the late 18th and early 20th century. Even the Volkswagen rear engine car was anticipated by a 1904 patent for a cart with the horse at the rear. George Valensi's patent lasted until 1971 because().
以下不属于结构化方法应遵循的基本原则的是
分析某地麻疹患者年龄构成，显示主要集中在学龄儿童(单峰)，提示
数据模型定义了数据库系统中数据组织、存储和管理必须遵循的规范，可以分为概念数据模型、数据结构模型和物理数据模型三大类。其中概念数据模型的典型代表是
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