试析SQL中EXISTS关键字的主要作用

The Important Roles of EXISTS Keywords in SQL

文章给出了在SQL编程中,使用EXISTS关键字完成对空集的判定,实现属于、包含、相等等语义,实现交、差和除等操作的具体方法,包括解题的基本思路、编程的核心步骤以及注意事项等诸多方面,易于理解与学习。针对普遍认为难以处理的具有全称量词或包含等语义的编程问题,文中用集合论中的基础知识,给出了一套简洁明了、浅显易懂的解题方法。

The EXISTS keywords are widely used in SQL programming, including how to determine whether a set is empty or not, how to express the semantics such as belonging to, inclusion, equivalence and so on, and how to implement the operations like intersection, except and division etc. All of these concrete methods are discussed in this paper. Furthermore, some other topics including the basic idea of solving problems, the essential steps during programming, and the points of attention are also discussed, in a way which is easy to understand and study, in this paper. Finally, using the basic knowledge of set theory, we present a set of concise and easy-to-understand methods for solving the programming problems involved the universal quantifier or the inclusion semantics, which are thought to be hard to deal with in the existing literature.