摘要 : In addition to data, database systems store various kinds of information about their data. Examples are: class hierarchies, to define the various data classes and their relationships; integrity constraints, to state required relat... 展开 In addition to data, database systems store various kinds of information about their data. Examples are: class hierarchies, to define the various data classes and their relationships; integrity constraints, to state required relationships among the data; and inference rules, to define new classes in terms of known classes. This information is often referred to as intensional information (the data are referred to as extensional information). There have been several independent research works that suggested ways by which intensional information may be used to improve the conventional (extensional) database answers. Although each of these efforts developed its own specific methods, they all share a common belief: database answers would be improved if accompanied by intensional statements that describe them more abstractly. We study and compare the various approaches to intensional answers by using various classifications; we examine their relative merits with regard to key aspects; we discuss remaining issues; and we offer new research directions. 收起