Chapter 4 - Entity Relationship Modelling (ERM) Flashcards
Entities
🐷Refers to entity set and not to single entity occurrence
🐷Corresponds to table and not to row in relational environment
🐷In Chen and Crow’s Foot models, entity is represented by rectangle with entity’s name
🐷The entity name, a noun, is written in capital letters
Attributes
- Characteristics of entities
- Chen notation: Ovals containing its names connected to entity rectangle with a line.
Crow’s foot notation: attributes written in attribute box below entity rectangle
Required attribute
Must have a value
Optional attribute
may be left empty
Domain
Set of possible values for an attribute
- Attributes may share the same domain
Identifiers
One or more attributes that uniquely identify each entity instance
Composite identifier
Primary key composed of more than one attribute
Which attribute can be subdivided?
Composite attribute
Which attribute cannot be subdivided?
Simple attribute
Single-value attribute
can have only a single value
Multivalued attribute
can have many values
Derived attribute
- value may be calculated from other attributes
- need not to physically stored within database
Participants
are entities that participate in a relationship
Connectivity
describe the relationship classification
Cardinality
expresses minimum and maximum number of entity occurrences associated with one occurrence of related entity
Existence dependence
Entity exists in database only when it is associated with another related entity occurrence
Existence independence
can exist apart from one or more related entities
-referred to as a strong or regular entity
Weak (non-identifying) relationships
Exists if PK of related entity does not contain PK component of parent entity
Strong (identifying) relationships
Exists when PK of related entity contains PK component of parent entity
Weak Entities
- Existence-dependent
- Primary key partially or totally derived from parent entity in relationship
Optional participation
– One entity occurrence does not require corresponding entity occurrence in particular relationship
Mandatory participation
– One entity occurrence requires corresponding entity occurrence in particular relationship
Relationship Degree
the number of entities or participants associated with a relationship
Unary relationship
Association is maintained within single entity
Binary relationship
Two entities are associated
Ternary relationship
Three entities are associated
Recursive Relationships
Relationship can exist between occurrences of the same entity set
–Naturally found within unary relationship
Associative (Composite) Entities
- Also known as bridge entities
- Used to implement M:N relationships
- Composed of primary keys of each of the entities to be connected
- May also contain additional attributes that play no role in connective process
relational schema
table structure
iterative process
A process based on repetition of steps and procedures.
Building an ERD usually involves the following activities:
- Create detailed narrative of organization’s description of operations
- Identify business rules based on description of operations
- Identify main entities and relationships from business rules
- Develop initial ERD
- Identify attributes and primary keys that adequately describe entities
- Revise and review ERD
