tracy07280917
请google scholar 谷歌学术 -- database或wikipedia 维基百科 -- database以下为部分节选A database consists of an organized collection of data for one or more uses, typically in digital form. One way of classifying databases involves the type of their contents, for example: bibliographic, document-text, statistical. Digital databases are managed using database management systems, which store database contents, allowing data creation and maintenance, and search and other access.Database architecture consists of three levels, external, conceptual and internal. Clearly separating the three levels was a major feature of the relational database model that dominates 21st century databases.The external level defines how users understand the organization of the data. A single database can have any number of views at the external level. The internal level defines how the data is physically stored and processed by the computing system. Internal architecture is concerned with cost, performance, scalability and other operational matters. The conceptual is a level of indirection between internal and external. It provides a common view of the database that is uncomplicated by details of how the data is stored or managed, and that can unify the various external views into a coherent whole.
曼特宁先森
Computer have come ato our home silently. Because of study, relax and so many reasns, computers appear in many people's homes. A few years ago, computer is just a daydream to me, now however, it has become my close friends. Through computer we can search a lot information easily at home, instead od losing ourselves in books's ocean. There's no double of that benefit of computer is huge. Because of it, we commmunicate many more, and we just need to pay a little to talk with our friends in foreign countries, and also get rid of the horrible telephone bills. At the same time, the shopping online is also convenient to us very much, all the trasations is just like pressing a key, and the thing you need will come to you in a short time. Saving your time is saving your life. The home computer really saves your life. SOHO is a new occupation because of the popular of home computer. Don't you want to work at your comfortable home? However, everythings has two sides. The benefit is so big that its adverse consequences also can't be forgotten. Many more recluses come out because of extremely losing themselves in computer. They get addicted deeply, and escape from the real world. And a lot of students can't control themselves from computer games. The poor thing is computer not only distory human's mental, ita slos let people's pphysic go bad. Eyeside will became worse and worse, and the changes of catching cancer will became higher and higher too. Unquestonably, the develop of computer brings plenty of convenient to us. Because of it, our life change a lot. On the other hand, we must also know the terrible result come with it. Many people can't stand up against the lure of it, and become the slave of computer. The computer becomes a strong drug and makes they addicted. So we must promote the right use of computer to all the people, let the benefit service us and the adverse affect get away from us.
飛8469682648
There is no denying that a computer offers us convenience and benefits. In spite of its advantages, its disadvantages are also evident. First, more and more students lose themselves in computer games, which is not only waste of money but also waste of time. Second, there is much false information on the Internet, and sometimes it is very difficult to distinguish what is true and what is false. Some people even commit crimes through computers.
千羽YUmi
嗨，我叫魏嘉哲，我喜欢吃米饭但是我不喜欢吃西红柿。我的朋友喜欢吃Hi,mynameisWeiJiazheIliketoeatrice,butIdonotliketoeattomatoes.Myfriendsliketoeatchickenbuthedoesnotliketoeatcarrots.鸡肉但是他不喜欢吃胡萝卜。
阿优米酱
Hello,I am Dale.I like hamburgers and apples for breakfast.But I don't like bread and milk.For lunch,I like rice、fruit salad and chicken.I don't like noodlles.For dinner,I like porridge and vegetable.And I don't like eggs.层次模型特点：用一颗“有向树”的数据结构来表示表示各类实体以及实体间的联系，树中每一个节点代表一个记录类型，树状结构表示实体型之间的联系。网状模型特点：用网络结构表示实体类型及其实体之间联系的模型。层次模型优缺点：结构清晰，结点间联系简单，只要知道每个结点的双亲结点，就可以知道整个模型结构。但是查询子女结点必须通过双亲结点，因为层次模型对任一结点的所有子树都规定了先后次序，这一限制隐含了对数据库存取路径的控制。网状模型优缺点：能够更为直接地描述现实客观世界。可表示实体间的多种复杂联系。但是结构比较复杂，其数据定义语言(DDL)、数据操作语言(DML)复杂，用户不容易使用。而且应用环境越大，数据库的结构就变得越复杂，不利于最终用户掌握。扩展资料网状模型取消了层次模型的不能表示非数状结构的限制，两个或两个以上的结点都可以有多个双亲结点，则此时有向树变成了有向图，该有向图描述了网状模型。层次模型的数据操纵主要有查询、插入、删除和更新。进行插入、删除、更新操作时要满足层次模型的完整性约束条件。进行插入数据时，如果没有相应的双亲结点值就不能插入它的子结点值。进行删除数据时，如果删除双亲结点值，则相应的子结点值也被同时删除
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