Enhanced Entity Relationship Schema Flashcards
IS A
Type of relationship (employee is a user)
Inheritance
One entity is derived from another (subclass).
Shown via C connection.
Specialisation (top-down)
Attributes/relationships only apply to certain but not all members of entity type
Generalisation
Main idea is to suppress differences between entity types
Common features
Attribute-defined subclass
Can define what will be inherited.
Outside of connection line.
Predicate-defined subclass
The rule that defines it / attribute extended
If subclass is professional, then Job Type (the attribute-defined subclass) = “Professional”
Disjoint Constraints
A subclass of a specialisation is disjoint ; comparing the connected subclasses
Disjoint
Either one or the other (d)
Overlapping
Could be more than one (o)
Double line
Must be classified (has to either be salaried or hourly-paid)
Single line
Does not need to be an employee or a professional