CH 5 - ER Modelling Flashcards
rember to dtraw er diagrams with chen and crows foot
What is the definition of a model according to Webster’s Dictionary?
A model is a “description or analogy used to visualize something that cannot be directly observed.”
What is a data model?
A data model is a relatively simple representation of complex real-world data structures.
what are the degrees of abstraction
-conceptual
-internal
-external
-physical
What does the conceptual model in data modeling represent?
It provides a global view of data,
serves as the basis for identifying and describing main data items,
and is typically represented using an ERD (Entity-Relationship Diagram). It is hardware and software independent.
What is the internal model in data modeling?
The internal model represents the database as seen by the DBMS, adapts the conceptual model to a specific DBMS, and is software dependent.
What is the external model in data modeling?
The external model represents users’ views of the data environment, provides subsets of the internal view, and ensures the adequacy and security of the conceptual model.
What does the physical model in data modeling represent?
The physical model is the lowest level of abstraction and is software and hardware dependent. It requires the definition of physical storage devices and access methods.
What does the Entity-Relationship (E-R) Model represent?
-conceptual view of the database
-showing the main components like entities and attributes and relationships
In the E-R Model, what is an entity?
An entity corresponds to an entire table (not a row) and is represented by a rectangle in an ERD.
What are attributes in the E-R Model?
Attributes are characteristics of entities and relationship
Their domain is the set of possible values they can have.
Primary keys are underlined.
er attributes
simple
composite
single valued
multi valued
derived
What is a simple attribute?
an attribute that cannot be subdivided, such as age, sex, or marital status.
What is a composite attribute?
A composite attribute can be subdivided into additional attributes, like an address can be divided into street, city, and zip code.
What is a single-valued attribute?
it can have only one value, such as a person having one social security number.
What is a multi-valued attribute?
A multi-valued attribute can have many values, such as a person having several college degrees.
What is a derived attribute?
A derived attribute can be calculated using an algorithm, like age being derived from the date of birth.
What is a relationship in the E-R Model?
A relationship is an association between entities, where connected entities are called participants.
Relationships operate in both directions and are described by connectivity (e.g., 1:1, 1:M)
cardinality.Expresses number of entity occurrences associated with one occurrence of related entity
relational strengths
existence dependence. Entity’s existence depends on existence of related entities. Existence-independent entities can exist apart from related entities
weak(non - identifying). One entity is existence-independent on another. PK of related entity doesn’t contain PK component of parent entity
strong (identifying).One entity is existence-dependent on another. PK of related entity contains PK component of parent entity
What is optional relationship participation?
Entity occurrence does not require a corresponding occurrence in related entity
Shown by drawing a small circle on side of optional entity on ERD
What is mandatory relationship participation?
Entity occurrence requires corresponding occurrence in related entity
If no optionality symbol is shown on ERD, it is mandatory
What is a weak entity?
A weak entity is one that is existence-dependent on another entity .
has a primary key that is partially or totally derived from the parent entity.
What is relationship degree in the E-R Model? nb draw them
indicates the number of associated entities e.g
unary (single entity associated),
binary (two entities),
ternary (three entities).
What are composite entities in the E-R Model?
are used to bridge M:N relationships
and are composed of primary keys of each entity needing connection.
What is an entity supertype and subtype? nb draw them
its a generalization hierarchy whereby :
An entity supertype is a higher-level entity that has shared attributes,
while subtypes are lower-level entities with unique attributes.
Relationships can be disjoint (non-overlapping ‘G’) or overlapping ‘Gs’ , indicated by specific symbols in ERDs.
What are the Chen and Crow’s Foot notations?
Chen notation was instrumental in moving conceptual design into practical database design,
while Crow’s Foot notation cannot detail all cardinalities but is widely used for simplicity in ERDs.
what is Rein85 and IDEF1X
Rein85 is similar to crows foot and operates at higher level of abstraction.
while
IDEF1X uses fewer symbols and is a derivative of ICAM
