ER_Modeling Flashcards
What is a Required Attribute?
it is an attribute that must have a value at the time the instance is created. NULL can not be a value
What is an Optional Attribute?
it is an attribute that does not require a value at the time the instance is created. There can be a NULL value
What are Domains?
they are the set of possible values that an attribute can contain
How are Domains written?
(<mininum>, <maximum>)
(<value1>, <value2>, <valueN>)</valueN></value2></value1></maximum></mininum>
e.g QPA(0, 4)
What are Identifiers?
they are composed of one or more attributes that uniquely identify one instance from the other
How are Identifiers represented in the ERD?
by placing an underline beneath the attribute
What are Composite Identifiers?
they are composed of two or more attributes that uniquely identify one instance from the other
What is a Composite Attribute?
it is an attribute that can be subdivided into several new attributes
What is a Simple Attribute?
it is an attribute that can not be subdivided into several new attributes
What is a Single-valued Attribute?
it is an attribute that can only have one value.
NOTE: a single-valued attribute is not necessarily a simple attribute
What are Multivalued Attributes?
they are attributes that can have many values, and they are represented in an ERD by connecting a double line from the multivalued attribute to the related entity.
Are multivalued attributes allowed in the RDBMS?
no
What are the two methods in implementing multivalued attributes to the RDBMS
a) create new attributes within the entity for the multivalued attribute
b) create a new entity using the components of the multivalued attribute
What are Derived Attributes?
they are attributes whose values are calculated based on other attributes
How are derived attributes represented in ERD’s?
using a dashed line to connect the attribute to the related entity
What is a Cardinality?
it is the minimum and maximum amount of times that an entity can occur
NOTE: a DBMS can not handle the implementation of a cardinality instead the cardinality is implemented in the application program
What are Weak (non-identifying) Relationships?
this occurs when a related entity’s primary key does not contain the primary key of its parent entity
What are Strong (identifying) Relationships?
this occurs when a related entity’s primary key also contains the primary key of its parent entity
NOTE: the order in which the tables are created and loaded is important because the related entity depends on the existence of its parent entity
What is a Weak Entity?
it is an entity type whose existence depends on some other entity type
NOTE: it does not typically have its own identifier instead it relies on an attribute (partial identifier) of its strong related entity to identify itself
What is a Strong Entity?
it is an entity that exists independently of other entity types
What is an Optional Participation?
this exists when an entity does not require the occurrence of another entity type, so its minimum cardinality is 0
What is a Mandatory Participation?
this exists when an entity requires the occurrence of another entity type, so its maximum cardinality is 1
What is a Relationship Degree?
this indicates how many entities are associated with a relationship
What is a Unary Relationship?
this exists when an association occurs within a single entity
What is a Binary Relationship?
this exists when two entities are associated in a relationship, this is the most common form of relationship
What is a Ternary Relationship?
this exists when there is an association among three different entities in a relationship
