One of the fundamental features of deductive databases is the deductive capabilities in which new facts may be derived from facts that were explicitly introduced. Deductive capabilities in such databases are implemented by use of intensional databases (IDB) and an inference rule. Most researches investigate function-free databases, thus excluding the possibility of occurrence of Skolem functions in IDB. In general, an existentially quantified variable in Prenex normal form of IDB may represent a null value, and cause a lot of overhead to query processing. On the other hand, some controlled occurrences of existential quantifiers in IDB can enhance the expressive power of deductive databases without introducing a null value. This usage of existentially quantified variables corresponds to the division operation of relational algebra. Consider a Ph/D candidate view that represents students who have taken every core course, which is formulated as below.
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