FindingEntities寻找实体.ppt

1、Finding Entities,What is an Entity,An entity is a person, place, object, event, or concept about which the organization wishes to record data. Entities should be things that are readily recognized by anyone working in the enterprise.,Some Entity Rules,An Entity 1. Must be within the scope of the sys

2、tem 2. Must have at least one non-key attribute,Where to Begin,To begin, we must have some form of information that we can use in our analysis. Collect information about the major business functions and processes. We can use questionnaires, interviews, observations etc.,Where to Begin,Another good s

3、ource of information for data modeling can be obtained by collecting any forms that are used by the enterprise. These forms provide valuable information on what data is important to the enterprise and how it is grouped.,Where to Begin,Once we have a concise picture of the processes we are going to m

4、odel, we can begin the iterative process of finding the entities that are necessary to support the enterprise.,The Steps Involved,1. Identify the system information a. User interviews b. Paper and screen documents c. Previous system documentation,The Steps Involved,2. Identify entities a. Analysis o

5、f English grammar - Entity-Relationship Diagrams b. Heuristics c. Normalization process,Analyze the Grammer,Using the table on the next slide, find the entities, attributes and relationships from the description and other information we have gathered about the enterprise.,Use this table to start,Sen

6、tence Restructuring,Next we do some sentence restructuring: Convert any sentence in the following form: “There are . . . X in Y” into “Y has . . . X”,Sentence Restructuring Example,“There are more than 200 employees in every department.” “Every department has more than 200 employees.”,X,Y,X,Y,More R

7、estructuring,If the English sentence says, “The X of Y is Z” Y is an Entity X is either an Attribute or an Entity, depending on what type of “Z” it is. If “Z” is a proper noun, then X is a Relationship between Y and Z Otherwise, X is an Attribute of Y,More Examples,The birthdate of the student is 10

8、/22/71. (x) (y) (z) Attribute Entity not a proper noun The advisor of John Smith is Dr. Greene. (x) (y) (z) Relationship Entity Proper noun,Candidate Entities,The preceeding steps have provided a list of Candidate Entities. Now we need to apply a few more rules to ensure we have only those entities

9、that are essential to the enterprise.,Paring Down the List,First, eliminate any duplicate entities. These can be entities with different names but representing the same thing. For example Patron and Customer. These may both represent the same entity. Second, eliminate and entities that are not impor

10、tant to the enterprise. This may require some analysis and work with the user/end customer.,Now add some Heuristics,When is an entity an entity? Any entity with only one evident instance (e.g. The National Basketball League, NHL, NFL etc.) is not a good entity because it is modeling business context or the business itself. If one ENTITY is in the center of everything and is connected to everything else it is probably the view point or context rather than an entity.,Finely,Remember that eventuall