Entity Relationship Modeling: Lesson 5 Flashcards
What does entities refer to?
It refers to an entity set and not a single occurrence
List the 8 types of attributes
Required attributes, optional attributes, identifiers, composite identifiers, composite attribute, simple attribute, single-value attribute, multi-valued attribute
What is a required attribute?
It is a attribute that must have a value
What is an optional attribute?
It is a attribute that does not require a value, example: STU_Middle Name
What is a domain?
It is a set of possible values for a given attribute
What is a identifier(primary key)?
It is one or more attributes that uniquely identify each entity instance.
What is a composite identifier?
It is a primary key that is composed of more than one attribute
What is a composite attribute?
It is a attribute that can be further subdivided to yield additional values. Address can be subdivided into street, city, state and zip code
What is a simple attribute?
It is a attribute that cannot be subdivided
What is a single valued attribute?
It is a attribute that only has a single value, example: SE-08-02-189935 is single-valued, but it is a composite attribute because it can be subdivided into
the region in which the part was produced (SE),
the plant within that region (08),
the shift within the plant (02),
and the part number (189935).
What is a multi-valued attribute?
It is an attribute that can have many values, example: A person may have several college degrees.
What is a derived attribute?
It is an attribute whose value is calculated from other attributes
What does the term existence dependence mean?
An entity is said to be existence-dependent if it can exist in the database only when it is associated with another related entity occurrence.
Relationship “EMPLOYEE claims DEPENDENT”
The DEPENDENT entity is clearly existence-dependent on the EMPLOYEE entity because it is impossible for the dependent to exist apart from the EMPLOYEE in the database.
Explain a weak( non identifying) relationship?
Exists if the primary key of the related entity does not contain a primary key component of the parent entity.
Explain strong(identifying) relationships
Exists when the primary key of the related entity contains a primary key component of the parent entity.
