Abstract
A lossless database scheme with full implicational dependencies is considered. The following condition is necessary in order that restricted projections of the representative instance could be expressed in relational algebra (i.e., in first-order logic): The dependencies of the database scheme are equivalent to a single join dependency and some equality-generating dependencies. An important special case is when the database scheme has only full tuple-generating dependencies. In this case, restricted projections of the representative instance can be expressed in relational algebra if and only if the following condition is true: The dependencies of the database scheme are equivalent to a single joint dependency. Testing this condition is decidable. This result also applied to the class of Horn-clause programs consisting only of typed rules with one predicate symbol R (and without function symbols). A program consisting of rules of this form is equivalent to a nonrecursive program if and only if it is equivalent to a project-join mapping. Finally, it is also shown that a tableau mapping is idempotent if and only if it is a project-join mapping.
Original language | English |
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Pages (from-to) | 1-22 |
Number of pages | 22 |
Journal | SIAM Journal on Computing |
Volume | 17 |
Issue number | 1 |
DOIs | |
State | Published - 1988 |