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Creating a database logical design is one of the first important steps in designing a database There are four logical database models that can be used hierarchical network relational or object-oriented 1Hierarchical The first important logical database model Still found in many mainframe-based legacy systems Records are arranged in a top-down parentchild structure Each child can be related to only one parent 2Network Developed to overcome the limited scope of the hierarchical model Similar to the hierarchical model only theres no parentchild distinction Any record type may be associated with any number of different record types 3Relational Data are represented in the form of tables with rows and columns Attributes are listed in columns entity instances are represented by a row entry 4Object-Oriented Data attributes and operations on the attributes are encapsulated within objects Objects can inherit properties can be generalized to form new objects and are reusable The O-O model is a fairly new way of looking at things For this paper we will be using the relational model With that decided we can start on the three steps in logical design 1Construct an E-R diagram to a set of relations two-dimensional tables 2Represent each entity type as a relation 3Represent each relationship via primary and foreign keys This can be done by following a couple of easy rules Normalize the relations to ensure a well-conditioned model Merge the relations to eliminate redundancies First introduced in 1970 by E F Codd the relational database model represents data in the form of tables or relations Mathematically based yet relatively straightforward and easy to understand Consists of three components 1Data Structure via tables 2Data Manipulation via SQL for example 3Data Integrity via formal business rules Relations A relation is a named two-dimensional table of data The columns of the relation represent the attributes of the entity the relation is modeling Each row reflects a specific entity instance Relations have the following properties Entries in columns
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