Mobile Communication Essay

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Topic 3: The Relational Database Model Performance Criteria Evaluate the basic relational model’s basic components: entities, attributes and relationships among entities. Compare and contrast various keys. Explore integrity rules and explain the two types of integrities Design entities and their attributes. Continue… Examine the importance of data dictionary. Discuss ERD components and how it affects database design and implementation. Investigate data redundancy issues in the “Small invoicing system” Draw ERD diagram using Crow’s Foot model " A Logical View of Data Relational model View data logically rather than physically Table Structural and data independence Resembles a file conceptually Relational database model is easier to understand than hierarchical and network models Tables and Their Characteristics Logical view of relational database is based on relation Relation thought of as a table Table: two-dimensional structure composed of rows and columns Persistent representation of logical relation Contains group of related entities (entity set) Keys Each row in a table must be uniquely identifiable Key: one or more attributes that determine other attributes Key’s role is based on determination If you know the value of attribute A, you can determine the value of attribute B Functional dependence Attribute B is functionally dependent on A if all rows in table that agree in value for A also agree in value for B Types of Keys Composite key Composed of more than one attribute Key attribute Any attribute that is part of a key Superkey Any key that uniquely identifies each row Candidate key A superkey without unnecessary attributes Types of Keys (cont’d.) Entity integrity Each row (entity instance) in the table has its own unique identity Nulls No data entry Not permitted in primary key
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