The semijoin algebra and the guarded fragment
نویسندگان
چکیده
The semijoin algebra is the variant of the relational algebra obtained by replacing the join operator by the semijoin operator. We discuss some interesting connections between the semijoin algebra and the guarded fragment of first-order logic. We also provide an Ehrenfeucht-Fräıssé game, characterizing the discerning power of the semijoin algebra. This game gives a method for showing that certain queries are not expressible in the semijoin algebra.
منابع مشابه
The Semijoin Algebra
When we replace, in the classical relational algebra, the join operator by the semijoin operator, we obtain what we call the semijoin algebra. We will show that, when only equi-joins are used, the semijoin algebra is equivalent with the guarded fragment of first-order logic, and thus it inherits many of the nice properties of the latter logic. When more general theta-joins are used, however, we...
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عنوان ژورنال:
- Journal of Logic, Language and Information
دوره 14 شماره
صفحات -
تاریخ انتشار 2005