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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]

Time-by-Example (TBE) is a user-friendly query language designed specifically for historical relational databases. It follows the graphical structure and the example query concept of QBE, and employs the hierarchical arrangement of subqueries of Abe and STBE. Similar to STBE, TBE handles set- and simple-valued attributes. In addition, to handle time, TBE is capable of manipulating triplet- and set-triplet-valued attributes. The underlying data model used in TBE is an extended relational data model in which nonfirst normal form relations and attribute time stamping (in contrast to tuple time stamping) are used. A triplet is a 3-tuple whose components are the lower and upper bounds of a time interval and a value valid over the interval. A triplet is used as a time- stamped value of a time-dependent attribute. Set-valued time-dependent attributes are modeled by sets of triplets. To process TBE queries and to define a historical relational algebra (HRA), standard operators of the relational algebra and the pack/unpack operators of [21] are augmented by triplet-decomposition, triplet- formation, slice, and drop-time operators. Methodologies for translating TBE queries into HRA expressions and for constructing their parse trees are presented. [ABSTRACT FROM AUTHOR]