Transcript 04_ldm1 - University of Central Florida

IMS 4212: Introduction to Data Modeling
Introduction to Data Modeling—Topics
•
•
•
•
Introduction to Data Modeling
Information elements
Introduction to Entities, Attributes, and Relationships
Basic notation
– Chen
– Alternative
• More on attributes
Dr. Lawrence West, Management Dept., University of Central Florida
[email protected]
1
IMS 4212: Introduction to Data Modeling
What is Data Modeling?
• Data modeling is a step in the process that begins with
the planning phase of Information Engineering and
ends with construction of the physical database
Information
Systems
Planning
Information
Elements
Entities
Attributes
Relationships
Rules
Physical
Database
Data Modeling
Dr. Lawrence West, Management Dept., University of Central Florida
[email protected]
2
IMS 4212: Introduction to Data Modeling
What is Data Modeling (cont.)
• Data Modeling is a process of requirements
identification, documentation, and revision that results
in a finished DB design
– Process begins with gross identification of basic DB
components
– Design is refined according to rules for storage and
retrieval efficiency
• Finished DB design is converted to the physical DB
– Some DB design tools make the conversion
automatically
Dr. Lawrence West, Management Dept., University of Central Florida
[email protected]
3
IMS 4212: Introduction to Data Modeling
Information Elements
• IS Design involves interviews with clients
– Clients don’t understand our terminology or DB
concepts (or they wouldn’t need us!)
– We probably don’t understand much of theirs
– Examine forms and reports
• Interviews will result in a collection of "Information
Elements" (my term)
– Lists of items of concern to the client
– Items that crop up in interviews
– Items you recognize from your experience
Exercise
Dr. Lawrence West, Management Dept., University of Central Florida
[email protected]
4
IMS 4212: Introduction to Data Modeling
Information Elements (cont.)
• Task is to determine which part of a data model the
different information elements fit
– Entity
– Attribute
– Relationship
– Business rule
– System input or output
– None of the above (irrelevant)
• Our determinations generate the base data model
• Further analysis modifies and extends the data model
to its final form
Dr. Lawrence West, Management Dept., University of Central Florida
[email protected]
5
IMS 4212: Introduction to Data Modeling
Entities
• "A person, place, object, thing, event, or concept about
which the organization wishes to maintain data"
• Examples from the university's database might be
STUDENT, CLASS, and PROFESSOR
• Each entity in the final data model will become a table
in the physical database
• It is important to distinguish between entities and
attributes of an entity
– Distinction may change with perspective
• We will also create new entities as we refine our data
model
Dr. Lawrence West, Management Dept., University of Central Florida
[email protected]
6
IMS 4212: Introduction to Data Modeling
Occurrences
• "Occurrences" of an entity are individual instances of
the entity
– You are an occurrence of the STUDENT entity
– I am an occurrence of the FACULTY entity
• Occurrences correspond to records in the database
• Take care not to confuse occurrences with entities
– Some authors use the term “Entity Set” to imply that
the Entity is a collection of occurrences
Dr. Lawrence West, Management Dept., University of Central Florida
[email protected]
7
IMS 4212: Introduction to Data Modeling
Defining Entities
• It is amazingly important to explicitly define what is
meant by each entity
• What is contained in the following entities?
– Customer
− Order
– Sale
− Employee
• Entity descriptions become part of the DB
documentation (description property in SQL Server)
• You cannot assume that developers using the DB will
have the save vision for the meaning of an entity that
you do
Dr. Lawrence West, Management Dept., University of Central Florida
[email protected]
8
IMS 4212: Introduction to Data Modeling
Defining Entities (cont.)
• (One occurrence of this entity represents…) “A person
or organization that has purchased products from the
company or who has inquired about purchasing
products” (Customer)
• … “A person that has signed an employment agreement
with the company including former employees.
Excludes applicants, contractors, and contractor
employees” (Employee)
• Try very hard to avoid using the entity name as part of
the definition.
Dr. Lawrence West, Management Dept., University of Central Florida
[email protected]
9
IMS 4212: Introduction to Data Modeling
Attributes
• "A property or characteristic of an entity that is of
interest to the organization"
• E.g., what characteristics of a STUDENT are of interest
to the University?
– SSN, First Name, Last Name, Major, DOB, …
• What characteristics are not of interest?
• What about Professors and Classes?
• What about your project?
• Attributes become fields in a record in the physical
database
Dr. Lawrence West, Management Dept., University of Central Florida
[email protected]
10
IMS 4212: Introduction to Data Modeling
Describing Attributes
• Attributes must be described just like entities
• What do these mean for a Product entity?
– Price
− Weight
– Supplier
− ProductID
• “Street address excluding apartment number or P.O.
Box where employee receives mail”
• “Shipping weight of one unit of product including
packaging in decimal fractions of a pound”
• “Current retail price that will be charged to a customer
purchasing this product”
• Use Attribute description in SQL table definitions
Dr. Lawrence West, Management Dept., University of Central Florida
[email protected]
11
IMS 4212: Introduction to Data Modeling
Entities and Attributes
• There can be ambiguity—depending on perspective—
in determining what should be an entity and what
should be an attribute
– UCF may have an attribute of STUDENT that contains
the high school each student graduated from
– The State of Florida Dept. of Education may consider
high schools to be an entity with its own attributes
• Refinement of the database may require that some
attributes be turned into new entities—watch for this as
we continue in the course
Dr. Lawrence West, Management Dept., University of Central Florida
[email protected]
12
IMS 4212: Introduction to Data Modeling
Naming Entities and Attributes
• Balance brevity with completeness
• No Spaces
– Order Detail → OrderDetail or Order_Detail
• No SQL Reserved Words
– Order → CustomerOrder
– Date → OrderDate, HireDate, BirthDate
– Time, Table, Insert, …
• My preference is for “Pascal Case”
– CustomerOrder
– LastInventoryDate
Dr. Lawrence West, Management Dept., University of Central Florida
[email protected]
13