Browsing by Author "Candan, Kasim S."
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Item An Algebra and Calculus for Multidatabases with Integrity Constraints(1998-10-15) Candan, Kasim S.; Subrahmanian, V.S.Litwin et. al. have developed a language called MSQL for query multidatabases. Subsequently, Grant, Litwin, Roussopolous and Sellis have developed a calculus and algebra associated with MSQL that facilitates querying and interoperation in a multidatabase environment. In this paper, we build upon their framework by assuming that a set of integrity constraints must be satisfied. Even though each individual database in a multidatabase may satisfy the integrity constraints, the entire multidatabase itself may not satisfy the constraints. We propose three new data retrieval notions based on whether the constraint semantics is ``naive'', ``skeptical'' or makes ``choices.'' We propose a semantics for these operations, and develop an algebra and calculus based on these operators. We prove that the algebra can be embedded within the calculus -- however, the calculus is strictly more powerful than the algebra. We study various algebraic properties linking the newly defined operators together and show how these algebraic properties can be used for query optimization. (Also cross-referenced as UMIACS-TR-94-86)Item A Unified Treatment of Null Values using Constraints(1998-10-15) Candan, Kasim S.; Grant, John; Subrahmanian, V.S.An important reality when studying relational databases is the fact that entries in relational tables may often be "missing" or only partially specified. The study of such missing information has led to a rich body of work on "null values." It was recognized early on that there are many different types of null values, each of which reflects different intuitions about why a particular piece of information is missing. Different relations (or even the same relation) could contain different types of null values; yet, very little work has been done on providing a unifying model that reasons with different types of nulls. In this paper, we use constraints to provide a unifying framework for the most common types of nulls. We show how viewing tuples containing null values of these types can be viewed as constraints, and how this leads to an algebra for null values. In particular, this algebra contains a unique operator (called the "compaction" operator) used to remove redundancies from null valued relations. We have studied various properties of this algebra. We have built a prototype implementation based on the null valued operators described here and conducted various experiments using this testbed. (Also cross-referenced as UMIACS-TR-95-47)