Entity Relationship Schema Flashcards
Entities
The Box
A thing which can be distinctly defined.
Relationship
Diamond
Association between entities.
Attribute
Oval
Common properties shared by all instances of an entity type.
Simple Attribute
Has one component, so a first name.
Complex Attribute
Have a structure
A full name would be a complex attribute.
Primitiveness
An attribute which will be stored in the system as data.
Degree
Number of entity types in a relationship, which can have a constraint.
Cardinality
One to many, many to many etc.
One person can have many emails.
Participation
A participation constraint specifies whether an entity must
be in the given relationship
Weak Entity Type
Cannot be identified with isolation.
Represented by double box.
The front desk -> cannot be identified.
The front desk at Ashton Theatre -> is now identified.
Strategies for ER design
Top-down - start with schema containing high-level abstractions and work down
Bottom-up - start with schema containing basic-level abstractions and work up
Inside-out - start with central concepts
Identifying Relationship
Used for weak entities to demonstrate that this relationship defines an entity
Double diamond.
Key Attribute
Oval with Line
Identifies the entity.
Derived Attribute
Dotted Circle
Does not exist in the physical database, but their values are derived from other attributes present in the database.
