Dbms

Notes on Chapter 2 of Elmasri & Navathe (5th ed.) Still Under Construction 2.1 entropy Models, Schemas, and Instances One fundamental characteristic of the entropybase approach is that it provides some train of info abstraction by hiding details of data storage that be irrelevant to database users. A data baffle ---a collection of concepts that can be used to describe the abstract/logical structure of a database--- provides the necessary means to bring home the bacon this abstraction. By structure is meant the data types, relationships, and constraints that should hold for the data. Most data sit arounds also include a set of basic operations for specifying retrievals/updates. Object-oriented data personates include the idea of objects having behavior (i.e., applicable methods) beingness stored in the database (as opposed to purely passive data). According to C.J. conflict (one of the leading database experts), a data model is an abstract, selfcontained, logical comment of the objects, operators, and so forth, that together constitute the abstract railcar with which users interact. The objects discontinue us to model the structure of data; the operators allow us to model its behavior. In the comparative data model, data is take careed as being organized in two-dimensional tables comprised of tuples of attribute values.



This model has operations such as Project, Select, and Join. A data model is not to be confused with its implementation, which is a physical realisation on a real machine of the components of the abstract machine that together constitute that model. Logical vs. physical!! There are other well-known data models that have been the basis for database systems. The bestknown models pre-dating the relational model are the hierarchical (in which the entity types form a tree) and the mesh topology (in which the entity types and relationships between them form a graph). Categories of Data Models (based on grad of abstractness): high-level/conceptual: (e.g., ER model of Chapter 3) provides a view close...If you want to get a full essay, order it on our website: Ordercustompaper.com



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