English Words and Data Bases: How to Bridge the Gap

ENGLISH WORDS AND DATA BASES: HOW TO BRIDGE THE GAP Remko J.H. Scha Philips Research Laboratories Eindhoven The Netherlands ABSTRACT If a q.a. system tries to transform an Eng- lish question directly into the simplest possible formulation of the corresponding data base query, discrepancies between the English lexicon and the structure of the data base cannot be handled well. To be able to deal with such discrepancies in a systematic way, the PHLIQAI system distinguishes different levels of semantic representation; it contains modules which translate from one level to another, as well as a module which simplifies expressions within one level. The paper shows how this approach takes care of some phenomena which would be problematic in a more simple-minded set-up. I INTRODUCTION If a question-answering system is to cover a non-trivial fragment of its natural input-language, and to allow for an arbitrarily structured data base, it cannot assume that the syntactic/semantic structure of an input question has much in common with the formal query which would formulate in terms of the actual data base structure what the desired information is. An important decision in the design of a q.a. system is therefore, how to embody in the system the necessary knowledge about the relation between English words and data base notions. Most existing programs, however, do not face this issue. They accept considerable constraints on both the input language and the possible data base structures, so as to be able to establish a fairly direct correspondence between the lexical items of the input language and the primitives of the data base, which makes it possible to translate input questions into query expressions in a rather straightforward fashion. In designing the PHLIQAI system, bridging the gap between free English input and an equally un- constrained data base structure was one of the main goals. In order to deal with this problem in a sys- tematic way, different levels of semantic analysis are distinguished in the PHLIQAI program. At each of these levels, the meaning of the input question is represented by an expression of a formal logical language. The levels differ in that each of them assumes different semantic primitives. At the highest of these levels,the meaning of the question is represented by an expression of the English-oriented Formal Language (EFL); this lan- guage uses semantic primitives which correspond to the descriptive lexical items of English. The prim- itives of the lowest semantic level are the prim- itives of the data base (names of files, attributes, data-items). The formal language used at this level is therefore called the Data Base Language (DBL). Between EFL and DBL, several other levels of mean- ing representation are used as intermediary steps. Because of the space limitations imposed on the present paper, I am forced to evoke a somewhat mis- leading picture of the PHLIQA set-up, by ignoring these intermediate levels. Given the distinctions just introduced, the problem raised by the discrepancy between the Eng- lish lexicon and the set of primitives of a …

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