Search

This paper proposes a relational calculus (RC/S) which uses set comparison and set manipulation operators to replace universal quantifiers and negations (with the exception of formulas of type ...R(x), i.e., "x is not in R"). We argue that, compared to the relational calculus (RC) of Codd, RC/S queries are easier to construct and easier to comprehend. We prove that the expressive power of RC is equivalent to the expressive power of RC/S, and give algorithms for translating an RC query into an RC/S query and vice versa. We define a safe RCIS query as one that has finite output and that can be evaluated in finite time. We then define a subset of RC/S queries, called RC/S*, and prove that RC/S* is safe. RC/S* is compared to the existing largest safe subsets of RC, i.e., the evaluable formulas and the allowed formulas. We give algorithms to transform any evaluable formula into an RC/S* query. Also, some RC/S* formulas that are not evaluable are given. RC/S* queries can be directly implemented using a graphical language similar to Query-by-Example (QBE). We briefly describe two different graphical languages that are equivalent, to the RC/S* in expressive power, and compare these languages to the QBE. [ABSTRACT FROM AUTHOR]