Entity-Relationship Model Flashcards
ER Model
Intuitive graphic representation facilitating the understanding of modeled information requirements
de facto standard for conceptual design of a database
3 basic concepts of E/R model
Entity
Relationship
Attribute
3 other concepts of E/R model
Cardinality constraint
Generalization
Identifier
Entity
Class of objects that have objects in common
Unique name, represented with box
Instance (of an entity)
Specific object belonging to the collection(class,set) that the entity represents
Relationship
Logical link between two or more entities
unique name, represented by diamond
How is an occurance of a relationship modeled
a tuple (p,c) is an occurance of birth where p is occurance of student and c is occurance of city
Are duplicates allowed in relationships?
No
Binary Relationship
Instance of relationship is a pair of entity instances
Ternary relationship
instance of relationship is triple of entity instances (employee, office and project all related by assignment)
Recursive relationships
relationship that binds an entity to itself
player from table plays with player from table, person marries person
Attribute
Property of entity or relationship
Composite attributes
aggregating simple attributes
street, city, zipcode —> address
Constraints in E/R
- instance of relationship must refer to instances of entities
- No duplicate relationships? page 18
- Constraints that are defined by E/R designer
1 and 2 implicit, 3 EXPLICIT
Cardinality Constraints
min/max values that can be associated with an instance of the entity/relationship to which the attribute belongs
