Thr Entity Relationship Model Flashcards
What does Entity-Relationship (ER) modelling facilitate?
Database design and implementation that accurately reflects the needs of an organisation or system.
Define entity type in the context of ER modelling.
A group of objects with the same properties, identified by an enterprise as having an independent existence.
What is an entity occurrence?
Each uniquely identifiable object of an entity type.
What is the purpose of the ER model?
To support a user’s perception of data and conceal technical aspects of database design.
What are the basic concepts of the ER model?
- Entity types
- Relationship types
- Attributes
What distinguishes a strong entity type from a weak entity type?
Strong entity types have independent existence; weak entity types are existence-dependent on another entity type.
What is a primary key?
A candidate key that is selected to uniquely identify each occurrence of an entity type.
What does the term ‘relationship type’ refer to?
A set of meaningful associations among entity types.
What is the degree of a relationship?
The number of participating entities involved in a relationship.
True or False: A recursive relationship involves the same entity type participating more than once.
True.
What is a composite attribute?
An attribute composed of multiple components, each with an independent existence.
What does a multi-valued attribute do?
Holds multiple values for each occurrence of an entity type.
Fill in the blank: An attribute that represents a value derivable from a related attribute is called a _______.
Derived attribute.
What is the notation used to represent a weak entity type?
A double rectangle.
What is meant by multiplicity in relationships?
The number (or range) of possible occurrences of an entity type that may relate to a single occurrence of an associated entity type.
What does a binary relationship involve?
Two participating entities.
How is a ternary relationship defined?
A relationship involving three participating entities.
What is a structural constraint?
Constraints that may be placed on entity types participating in a relationship.
What is the purpose of an ER diagram?
To visually represent how different pieces of data are related within a system.
What does the term ‘candidate key’ refer to?
A minimal set of attributes that uniquely identifies each occurrence of an entity type.
What is the significance of role names in relationships?
Role names clarify the purpose of each relationship.
What type of attribute is ‘StaffNo’ in a Staff entity?
An example of a simple (or atomic) attribute.
What is the maximum degree for relationships typically used in ER models?
Binary (two participating entities).
What should an entity be according to ER modelling?
- An object that will have many instances in the database
- An object composed of multiple attributes
- An object we are trying to model
What is a derived attribute example in ER modelling?
Duration derived from rentStart and rentFinish.
What is the notation for associating attributes with relationships?
Dashed line.
What is the alternative notation for displaying an ER model?
There are various notations, but specific examples were not provided.
What is the relationship type called that represents an association between two entities?
Has relationship.
What is the multiplicity of a one-to-many relationship?
One entity can relate to many occurrences of another entity.
True or False: A weak entity can be identified solely by its own attributes.
False.
What does the term ‘semantic net’ refer to in ER modelling?
A visual representation of relationships among entities.
What is an example of a relationship occurrence?
A uniquely identifiable association, which includes one occurrence from each participating entity type.
What is the multiplicity of the relationship between Newspaper and PropertyForRent?
Many-to-many (:) relationship
Each newspaper can advertise one or more properties, and each property can be advertised in zero or more newspapers.
What does the notation ‘1..*’ beside the PropertyForRent entity signify?
Each newspaper can advertise one or more properties for rent
This indicates mandatory participation of PropertyForRent in the relationship.
What does the notation ‘0..*’ beside the Newspaper entity signify?
Each property for rent can be advertised by zero or more newspapers
This indicates optional participation of Newspaper in the relationship.
What are the two types of restrictions on relationships in multiplicity?
Cardinality and participation
Cardinality refers to the maximum number of occurrences, while participation indicates whether all or some entities participate.
What is cardinality in the context of ER models?
Maximum number of possible relationship occurrences for an entity in a given relationship type
Examples include one-to-one (1..1), one-to-many (1..), and many-to-many (..*).
What does mandatory participation mean in ER models?
All entity occurrences must participate in a relationship
Represented as a minimum value of 1 in multiplicity ranges.
What does optional participation mean in ER models?
Only some entity occurrences participate in a relationship
Represented as a minimum value of 0 in multiplicity ranges.
What is a fan trap in ER modeling?
A situation where a relationship between entity types is ambiguous due to multiple 1:* relationships from the same entity
Example: Two 1:* relationships coming from the same Division entity.
What is a chasm trap in ER modeling?
A situation where a model suggests a relationship between entity types, but the pathway does not exist for certain occurrences
Example: A branch has staff overseeing properties, but not all properties are overseen.
What is the minimum multiplicity of the Client entity in the ternary Registers relationship?
0..*
Indicates that a Staff entity at a Branch may register zero or more Clients.
What is the multiplicity of the Branch entity when Staff/Client values are fixed?
1..1
Indicates that each Branch must have exactly one Staff member in the relationship.
What is the UML notation for representing a primary key attribute?
PK_AttributeName
Primary key attributes are indicated with {PK} in UML notation.
What does ‘DerivedAttributeName’ represent in UML notation?
An attribute that can be calculated from other attributes
Indicated with a forward slash (/) before the attribute name.
How is a multi-valued attribute represented in UML notation?
MultiValuedAttributeName {min..max}
Indicates the range of possible values for the attribute.
What does the notation ‘1..M’ signify in Chen’s notation?
One-to-many relationship
Indicates that one entity can be associated with multiple occurrences of another entity.
What is indicated by a double rectangle in Chen’s notation?
Weak entities
Strong entities are represented with a single rectangle.
What does the Crow’s Foot notation represent for a many-to-many relationship?
M:M
Indicates that multiple occurrences of one entity can be associated with multiple occurrences of another entity.
What should you use for developing ER models in this module?
UML notation
Other notations will not be accepted in assessments.
