Define entity in the ER model
A group of objects with the same properties. Can be both physical and conceptual
What is relationship
- meaningful associations among two or more entities
Define degree of relationship
the number of participating entity types in a relationship
define attribute
a property of an entity or relationship
Define candidate key
The minimal number of attribute(s) whose value(s) uniquely identify each entity occurrence
Define composite key
A candidate key consisting of 2 or more attributes
Define Primary Key
The candidate key that is selected to uniquely identify each occurrence of an entity type
what are the principle are choosing the primary key
― Attribute length
― Minimal number of attributes required
― The future certainty of uniqueness
Define the alternate key
any data that hasn’t been used in the primary key
Define weak entity type
An entity that is exixtence-dependant of some other entity type
Define multiplicity
the number of possible occurrences of an entity type that may relate to a single occurrence of an associated entity type through a particular relationship
What might the binary relationships be described as
- One to one (1:1)
- One to many (1:*)
- Many to many (:)
Describe the one to one (1:1) relationship
Exactly one professor is needed to manage a discipline and a professor could manage at most one discipline
Describe the One to Many (1:*) relationship
Exactly one academic staff is needed to tutor a single student. Each academic staff may tutor many students
Describe the Many to Many (:) relationship
Students register for multiple Courses and Courses have multiple Students
Define Cardinality
the maximum values for the multiplicity ranges on either side of the relationship.
― One-to-one (1:1)
― One-to-many (1:)
― Many-to-many (:*)
Define participation
the minimum values for the multiplicity ranges on either side of the relationship
― Optional participation (if 0)
― Mandatory participation (if 1 or more)
What is a Fan Trap?
A situation where two or more 1:* relationships fan out from the same entity, creating ambiguous paths
Give an example of a Fan Trap
Division – Branch – Staff where we cannot tell which branch a staff works at
What is a chasm Trap
A situation where optional relationships break the path between related entities
Give an example of a chasm trap
Branch – Staff – PropertyForRent when optional participation prevents linking branches to some properties
What are two possible problems in ER modelling?
Fan Traps and Chasm Traps
