Entity-Relationship Design. Information Level Design. TOP DOWN DATA ANALYSIS.
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Information Level Design
Computer systems are extremely complicated and cannot be developed without careful planning. The most common MIS is the Systems Development Life Cycle. This approach is to build a model of the information system based on the objectives and goals it must meet. This is called top down modeling.
The AITP Service Club wants to run a babysitting service. Customers call to request a sitter and the Club Coordinator assigns an employee to sit for the customer from a list of employees available for the particular day requested.
D1 | Customer
D2 | Employee
D3 | Jobs
A good E-R model has
A person, place, object, event, or concept about which the organization wishes to maintain data.
classes of people, objects or concepts about which we wish to store data.
A description or property of a given entity type.
A connection between entity instances in different entity classes
1. Review the conceptual description of the business area for nouns that describe the system.
2. Each entity type should have more than one potential instance.
3. Each entity type should have more than one attribute.
4. Each entity type should be relevant..
A relationship is a connection between records in one table and those in another.
Does not describe processing or change any data. Relationship names should be passive (ordered by).
Records in a table that have a relationship with another table may be restricted by optionality requirements.
Optional (0 allowed)
Mandatory (1 or more required)
A constraint should be mandatory only if the relationship must be known whenever a record is first entered. Most relationships are optional.
If a parent record is deleted then an optionality relationships can be maintained in several ways