Les 6 - Class diagram to ERD and other UML diagrams
De les gaat vooral over verschillende UML diagrammen, zoals een ERD of een class diagram.
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Activity case
Answers to activity case
Video 1:
Video 2:
Aantekeningen
ERDs
- A model is a depiction of a part of reality.
- Modeling data: the first step in designing database.
- We use Entity Relationship Diagrams (ERD) to model the structure and relations between data: data model.
- An ERD includes: Entity sets ( containing Attributes) and Relationship sets.
Building blocks of an ERD
- Entities
- An entity is an object in the real world that is distinguishable from other objects.
- Examples:
- the student: any specific student is distinguishable from other students.
- ANL02-2-1718: A specific course code distinguishable from other courses.
- the teacher: any specific teacher is distinguishable from other teachers.
- Attributes
- An entity is described using a set of attributes.
The values given for the attributes makes it distinguishable. - Example:
- Attributes for a student: First Name, Last Name, Student Number, email, … .
- An entity is described using a set of attributes.
- Entity-relations
- Business Rules
Entity Set
We are interested in a collection of entities: Entity Set.
- A Set of entities that share the same attributes.
- Example: Entity set “Student”: the set of all the students that are sharing the same attributes; name, std number, email, … .
Primary Key
A Primary Key is a minimal set of attributes whose values uniquely identify an entity in the set.
- Examples:
- Entity set “Student”: student number is the minimal set of attributes that uniquely can identify a student.
- Entity set “Student”: combination of name and address can also make a student unique, but it is not minimal.
Relationship sets
Relations: Entities are associated with each other through relations.
Relationship: A Relationship is an association among two or more entities.
- Examples:
- John works in pharmacy department.
- Working relates members from Employee to members from Department.
Relationship Set: A set of relationships involving the same entity sets is defined as a Relationship Set.
- Example: Collect all working relations between employees and departments. Then we will have “Works” Relationship Set.
Remember that a relation between sets is a subset of the cartesian product.
- Consider the example in the image above: the relationship set connects together pairs (but not all the possible pairs) of entities from both the entity sets.
- It is a subset of the cartesian product
- Relationship sets = relations among sets.
Cardinality
IE notation (Martin notation):
One-to-Many: “the relationship set associates one entity from one entity set to many entities from other entity set.”
Many-to-Many: “the relationship set associates many entities from one entity set to many entities from other entity set.”
Unary: A relationship set can be between entities of one entity set.
Binary: A relationship set can be between entities of two different entity sets.
Ternary: A relationship set can be between entities of three different entity sets.
Participation and Cardinality Key Constraints
- Problem: A department has at most one manager.
- Note: This means, given a department from entity set department we can uniquely identify its manager from Employee entity set.