A
A database is supposed to represent the interesting semantics of an application (the
It comprises basic data types and constructors for composed data types, (generic) operators to insert, manipulate, retrieve and delete instances of the actual data types of a database, implicit consistency constraints as well as (eventually) mechanisms for the definition of explicit consistency constraints that further reflect the miniworld semantics as viewed by the database system.
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As usual, types have to be defined before instances of them can be created (the collection of defined types --- sometimes together with the set of explicit consistency constraints --- forms the
We can therefore distinguish the following two classes of semantics: the semantics of the miniworld itself, the semantics of the miniworld as represented within the database.
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Let us assume that a database
However, the ultimate goal of database systems is to provide for concepts that allow to keep the semantic gap as small as possible and thus permit to represent most of the salient semantics in the database itself.
一个数据库应该完整地表示应用程序( 它包括 基本数据类型和组合数据类型的构造函数, P> ITEM> (通用)运算符,用于插入,操作,检索和删除数据库实际数据类型的实例, P > ITEM> 隐式一致性约束以及(最终)用于定义显式一致性约束的机制,这些机制进一步反映了数据库系统所查看的小世界语义。 P> ITEM> LIST> rn P> rn 和往常一样,必须先定义类型,然后才能创建它们的实例(已定义类型的集合---有时连同一组明确的一致性约束- -形成 因此,我们可以区分以下内容:两类语义: 小世界本身的语义, P> ITEM> 数据库中表示的小世界的语义。 P> ITEM> LIST> rn P> rn 让我们假设数据库 数据库系统的最终目标是提供一些概念,以使语义差距尽可能小,从而可以代表数据库本身中的大多数显着语义。 P>
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