430 CS Database Theory
Winter 2005 Lecture 3: A Fifty Minute Introduction to Data Modeling
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What is Data Modeling?
Answer: Modeling the data that will be in our database What data will be in our database?
The data needed to support our application The data needed to meet the requirements for our application Note that database design leads application design
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Figure 3.1 from Text Book
Aside
When we build a data model we are modeling the data content of the "real world" The real world is very complicated But what's a model, in this case:
"A simplified description of something complex"
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How do we do modeling?
Use a method called "Entity/Relationship (ER) Modeling"
The most popular technique for data modeling That is, it tries to capture the semantics (meaning) of the data In this case semantics refers particularly to rules about the data
Also, sometimes called "Semantic Modeling"
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Entities and Relationships
Entity
A distinguishable (type of) object E.g. Person, Book Has an identity (distinct from the data about it) An entity that serves to connect two or more other entities A piece of information that describes an entity E.g. Name, Title
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Relationship
Attribute or Property
Informal
All of these concepts are informal In particular, as a design evolves, things change:
Relationships become Entities (and vice versa) Attributes become Relationships
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A little more complexity
Derived Attributes and Relationships
Ones that can be derived from other information E.g. Age can be derived from Birth Date Ones built up from simpler attributes What you think it means A collection of one or more attributes of an entity that uniquely identify entity that E.g. Your SSN, the ISBN for a book
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Composite Attributes
Single-Valued and Multi-Valued Attributes
Key
Yet More Complexity
Cardinality
Property of a relationship: 1 to 1, N to 1, 1 to N, M to N Sometimes also called total participation An entity can only exist if it participates in a specified relationship One that doesn't have a key Defined by their relationship to other entities (called an identifying relationship)
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Existence Dependency
Weak Entity
ER Diagrams
Figure 3.14 from Text Book
Shows diagrammatics
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Company Database Example
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Data Requirements (page 53, Text Book)
The company is organized into departments. Each department has a unique name, a unique number, and a particular employee who manages the department. We keep track of the start date when that employee began managing the department. A department may have several locations. Etc., etc., etc.
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Figure 3.2 from Text Book
ER Diagram for Company database Figure 3.16 Same thing as a UML Class Diagram
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Assignment #1
Work up a (rough) set of data requirements and a (rough) ER diagram for a database that would be used to run (some aspect of) a book store
The diagram can be hand drawn Make sure your data requirements match your diagram You will probably have to make some choices about what data to exclude Try to keep it consistent, don't worry about modeling everything Take a real world problem and think about the kinds of data that might be important for that problem. Try your hand at a little data modeling I'm going to use this as a basis for you to implement a sample database application
Keep it small (about size of Company database)
Goals:
Hidden Agenda:
Due: Monday, January 24
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