Studying modern database languages one recognizes that there is a gap between language
features and theoretical foundations:
* Studies of the formal foundations exist for the relational data model but not
for the Entity-Relationship model, which is a model used by numerous practical
people. Also, most extensions of the Entity-Relationship model and other
semantic data models lack a precise formal description.
* Certain features of database query languages such as aggregation or grouping are
offered by the corresponding database systems but the theoretical foundations
in terms of algebras or calculi frequently neglect these features.
This text presents a comprehensive introduction to an extended Entity-Relationship model both on a conceptual and on a formal, mathematicallevel. In addition to the primitives given by the data model the text introduces a language for the formulation of constraints in order to restrict database states to consistent ones. The text explains an implementation of the approach chosen in the logic programing language PROLOG and discusses in this context the computational power of the proposedcalculus. The extended Entity-Relationship calculus is used to define the meaning of the relational query language SQL. A nice feature of the approach is that it becomes possible to prove language properties on a sound mathematical basis.
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Open Sources 2
Open Sources 2.0 is a collection of insightful and thought-provoking essays from today's technology leaders that continues painting the evolutionary picture that developed in the 1999 book Open Sources: Voices from the Revolution .