Entity Relationship Model Flashcards
Expressed in terms of entities in the business environment, the relationship (or associations) among those entities, and the attributes (or properties) of both the entities and their relationships
Entity-Relationship Model
Detailed, logical representation of the data for an organization or for a business area
Entity-Relationship Model
is a graphical representation of an E-R Model
E-R Diagram
Basic Symbols
A person, place, object, event, or concept in the user environment about which the organization wishes to maintain data (NOUN)
Entity
Naming and Defining Entity Types
Singular Noun
Specific to organization
Precise
What Should an Entity Be?
- An object that will have many instances in the database
- An object that will be composed of multiple attributes
- An object that we are trying to model
Entity Types
- Weak
- Strong
An entity that exists independently of other entity types
Strong Entity
An entity type whose existence depends on some other entity
type
Weak Entity
The relationship between weak entity type and its owner
Identifying Relationship
Property or characteristic of an entity type that is of interest to the organization
Attributes
An attribute of an entity that must have a value for each entity instance
Required Attribute
An attribute of an entity that may not have a value for every entity instance
Optional Attribute
An attribute that cannot be broken down into smaller components
Simple (or atomic) attribute
An attribute that can be broken down into components parts
Composite attribute
An attribute that may take on more than one value for a given entity instance
Multivalued attribute
attributes hold a single value for each record.
Single valued attribute
are physically stored in the database.
Stored Attribute
whose values can be calculated from related attribute values
Derived Attribute
An attribute (or combination of attributes) that uniquely identifies individual instances of an entity type
Identifier Attribute
Association representing an interaction among the
instances of one or more entity types that is of interest to the organization
Has a verb phrase name – present tense
Relationship
A meaningful association between (or among) entity types
Relationship Type
An association between (or among) entity instances where each relationship instance includes exactly one entity from each participating entity type
Relationship Instance
may be associated with many-to-many (or one-to-one) relationship, as well as with an entity
Attributes
An entity type that associates the instances of one or more entity types
Associative Entity
involves a rectangle with a diamond inside.
Associative Entity
Each entity in the relationship will have exactly one related entity
One-to-One
An entity on one side of the relationship can have many related entities, but an entity on the other side will have a maximum of one related entity
one-to-many
A relationship between the instances of a single entity type
Unary Relationship
Entities on both sides of the relationship can have many related entities on the other side
many-to-many
The number of entity types that participate in a relationship
Degree
A relationship between the instances of two entity types
Binary Relationship
A simultaneous relationship among the instances of three entity types
Ternary Relationship
The maximum number of instances of one entity that may be associated with each instance of another entity
Maximum Cardinality
The minimum number of instances of one entity that may be associated with each instance of another entity
Minimum Cardinality
Specifies the number of instances of one entity that can (or must) be associated with each instance of another entity
Cardinality Constraints
