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Unformatted text preview: Exam review -DBMS -relieves the programmer or end user from the task of understanding where and how the data are actually stored by separating the logical and physical views of the data Logical views presents the data as they would be perceived by end users Physical view shows how data are actually organized and structured on physical storage media Makes the physical database available for different logical views required by users Reduces data redundancy and inconsistency by minimizing isolated files in which the same data are repeated May not enable the organization to eliminate data redundancy but it can help control redundancy Eliminates inconsistency because helps organization to ensure that every occurrence of redundant data has the same values Uncouples programs and data, enabling data to stand on their own Access and availability of info will be increased and program development and maintenance costs reduced because users can program ad hoc queries of data in the database Enables organization to centrally manage data, their use, and security Relational DMBS The most popular type of DBMS Represents data as two-dimensional tables (called relations) Tables may be referred to as files Each table contains data on an entity and its attributes Microsoft access is an example of one Each table consists of a grid of columns and rows of data Each individual element of data for each entity is stored as a separate field, and each field represents an attribute for that entity Fields in a relational database are also called columns Rows are commonly referred to as records or tuples Each table in a relational database has one field that is designated as its primary key The primary key is the unique identifier for all the information in any row of the table and cannot be duplicated The foreign key is essentially a look-up field to look up data about the supplier of a specific part Relational database tables can be combined easily to deliver data required by users, provided that any two tables share a common data element Three basic operations are used to develop useful sets of data: select, project, and join ♦ The select operation creates a subset consisting of all records (rows) in the table that meet stated criteria ♦ The join operation combines relational tables to provide the user with more info that is available in individual tables ♦ The project operation creates a subset consisting of columns in a table, permitting the user to create new tables that contain only the information required Hierarchical and network DBMS Used by older systems A hierarchical DBMS models one-to-many relationships Network DBMS models many-to-many relationships No longer used for building new database applications Much less flexible that relational and do not support ad hoc, natural language Relational has much more flexibility in providing data for ad hoc queries,...
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This note was uploaded on 04/26/2009 for the course MIS 301 taught by Professor Mccleod during the Spring '08 term at University of Texas at Austin.
- Spring '08