E-R models are expressed using a single standardized set of universally accepted symbols.
An entity is something in the users’ work environment that the users want to track.
Entities of a given type are grouped into entity classes.
An entity class is described by the structure of the entities in that class.
An entity instance of an entity class is the representation of a particular entity and is described by the values of the attributes of the entity.
In E-R modeling, entities within an entity class may have different attributes.
In E-R modeling, an attribute may be either composite or multi-valued, but it cannot be both.
An identifier of an entity instance must consist of one and only one attribute.
A “composite identifier” is defined as a composite attribute that is an identifier.
An identifier may be either unique or nonunique.
E-R modeling recognizes both relationship classes and relationship instances.
Relationships do not have attributes.
A single relationship class involves only one entity class.
A binary relationship is a relationship based on numerical entity instance identifiers.
The degree of a relationship is expressed as the relationship’s maximum cardinality.
A relationship’s minimum cardinality indicates whether or not an entity must be involved in the relationship.
Relationships among instances of a single entity class are called redundant relationships.
A weak entity is an entity that cannot exist in the database without (and is logically dependent upon) another type of entity also existing in the database.
ID-dependent entities are a common type of weak entity.
All weak entities must have a minimum cardinality of 1 on the entity on which it depends.
Multi-valued attributes are represented in E-R diagrams by creating a new weak entity to represent the multi-valued attribute and creating a 1:N relationship.
Subtype entities are used to produce a closer-fitting model when an entity has sets of optional attributes.
Entities with an IS-A relationship should have the same identifier.
Inheritance in a generalization hierarchy means that the supertype entity inherits all the attributes of the subtype entity.
It is not important to document business rules during data modeling since they will be enforced by the application programs.
UML is intended for modeling and designing object-oriented programs and applications.
For database design, UML-style E-R diagrams must be treated very differently from traditional E-R diagrams because of their object-oriented background.
Maximum cardinalities are represented in UML-style E-R diagrams with the same notation (1:1, 1:N, N:M) as in traditional E-R diagrams.
In UML-style E-R diagrams, a weak entity is shown by placing a filled-in diamond on the parent of the weak entity.
One weakness of UML-style E-R diagrams is that there is no means of distinguishing between a weak entity that is ID-dependent and a weak entity that is not ID-dependent.
UML-style E-R diagrams allow for the existence of class attributes, which are attributes that pertain to the collection of all entities within that class not to the individual entity instances
UML-style E-R diagrams allow for three different levels of visibility of attributes.
UML-style E-R diagrams introduce object-oriented notation that is of limited practical value in traditional, relational database design.
The goal of data models is to strive to produce the most accurate model of the real world as possible.
