Semantics of Temporal Queries and Temporal Data

SEHANTICS OF TEHPORAL QUERIES AND TEHPORAL DATA Carole O. Hafner College of Computer Science Northeastern University Boston, MA 02115 Abstract This paper analyzes the requirements for adding a temporal reasoning component to a natural language database query system, and proposes a computational model that satisfies those requirements. A prelim- Inary implementation in Prolog is used to generate examples of the model's capabi Iltles. I. Introduction A major area of weakness in natural language (NL) interfaces is the lack of ability to understar~ and answer queries involving time. Although there is growing recognition of the importance of temporal semantics among database theoretlcians (see, for example, Codd [6J, Anderson [2L Clifford and Warren [41, Snodgrass [ i 5]), existing database management systems offer little or no support for the manipulation of tlme data. Furthermore (as we will see In the next Section), there is no consensus among researchers about how such capabilities should work. Thus, the developer of a NL interface who wants to support time-related queries cannot look to an underlying ~ for he!p. Currently available NL systems such as Intellect (SJ have not attempted to sugoort temporal queries, except in a trivial sense. In Intellect, users can ask to retrieve date attributes (e.o~, "When was Smith hired'?') or enter restrictions based on the value of a date attribute (e.g., "List the employees hired after Jan I, 1984"); but more complex questions, such as "How long has it been since Smith received a raise~ or "What projects did Jones work on last January?', are not 'Jnderstoo~ This Is a serious PraCtical limitation, since the intended users of NL systems are executives and other professionals who will require more sopffistlcated temporal capal)illtles. This report describes a model of temporal reasoning that is designed to be tncoroorated Into a NL query system. We assume that a syntactic component could be developed to translate explicit temporal references in English (e.g., "two years ago') into logical representations, and restrict our attention to the conceptual framework (including both knowledge structures and rules of inference) underlying such representations. Section 2 analyzes the requirements that the temporal model must satisfy: first describing some of the issues that arise tn trying to model time in a computer, then defining four basic semantic relattonsl~ips that are expressed by time attributes in databases, and finally analyzing the capat)tlites required to Interpret a variety of temporal queries. Based on this analysis, a computational model is described that satisfies many of the requirements for understanding and answering time-related database queries, and examples are presented that t l lustrate the model's calDabiltties. 2. Hodellng Temporal Knowledge Hodellng time, dasoite its olovlous importance, has proved an elusive goal for artificial Intelligence (AI). One of the first formal proposals for representing time-dependent knowledge in AI systems was the "situation calculus" described by I'lcCarthy a~l Hayes [I I]. That proposal created a paradigm for temporal reasoning based on the notion of an infinite collection of states, each reoresenting a single instant of time. Prepositions are defined as …

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