By Sikha Bagui
Info, Databases, and the software program Engineering ProcessDataBuilding a DatabaseWhat is the software program Engineering Process?Entity courting Diagrams and the software program Engineering lifestyles Cycle part 1: Get the necessities for the Database part 2: Specify the Database section three: layout the DatabaseData and knowledge ModelsFiles, documents, and knowledge ItemsMoving from three × five playing cards to ComputersDatabase versions The Hierarchical ModelThe community ModelThe Relational ModelThe Relational version and useful DependenciesFundamental Relational DatabaseRelational Database and SetsFunctional. Read more...
summary: facts, Databases, and the software program Engineering ProcessDataBuilding a DatabaseWhat is the software program Engineering Process?Entity courting Diagrams and the software program Engineering existence Cycle section 1: Get the necessities for the Database section 2: Specify the Database part three: layout the DatabaseData and information ModelsFiles, files, and knowledge ItemsMoving from three × five playing cards to ComputersDatabase versions The Hierarchical ModelThe community ModelThe Relational ModelThe Relational version and practical DependenciesFundamental Relational DatabaseRelational Database and SetsFunctional
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Extra resources for Database Design Using Entity-Relationship Diagrams, Second Edition
The merchant who keeps this file may well want to add information in the future. Suppose that the merchant wanted to add a telephone number. Would the merchant add a number to all 3 × 5 cards, or would the adding be done “as necessary”? If it were done as necessary, then some customers would have telephone numbers, and some would not. If a customer had no phone number, then the phone number for that customer would be “null” (unknown). 2. How will the file be organized? Imagine not three customers, but 300 or 3,000.
What does null mean? 5. Consider this customer file: Record 1: 77 A St Adams, A Record 2: Charles, X 555-1234 365 Broad St Harris, H Record 3: Pensacola FL 555-8214 555-5847 92 Adams Lane Elberta, AL Mobile AL What is wrong here? 3 MOVING FROM 3 × 5 CARDS TO COMPUTERS Let us return to our example of a merchant who kept his customer file on 3 × 5 cards. As time passed, the customer base grew and the merchant probably desired to keep more information about customers. From a data-processing standpoint, we would say that the techniques for storing and retrieval led to enhanced forms or cards, more fields, and perhaps better ways to store and find individual records.
If the company were such that employees might have multiple dependents and a dependent might be claimed by more than one employee, then the cardinality 28 • Database Design Using Entity-Relationship Diagrams would be many to many: Many employees relate to many dependents, abbreviated M:N (as in M of one side relates to N on the other side, and M and N are generally not equal). Optionality refers to whether one record may or must have a corresponding record in the other file. If the employee may or may not have dependents, then the optionality of the employee-to-dependent relationship is optional or partial.