Negation poses certain challenges for queries and searches. This paper deals with exclusionary queries implemented using the ISO database language SQL and a dialog-based interface and with retrieval searches involving negation. This research arose because instructors in database courses noticed a large proportion of students making mistakes on certain queries. The paper explores underlying comprehension issues and makes practical recommendations on identifying potential sources of error and avoiding incorrect or misleading results. Proposed actions include changes in general education and database training and encouraging implementation of the new SQL:1999 standard.
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