3.1 The ER MODEL Flashcards
is a conceptual model that represents the information structure of a problem domain in terms of entities and relationships.
entity-relationship model (ERM)
Who developed ERM
Dr. Peter Pin-Shan Chen
When was the ERM developed
1976
is a type of structural diagram for use in database design.
Entity Relationship Diagram (ERD)
What are the 3 basic elements of ERD
Entity
Attributes
Relationship
is a person, place, object, event, or concept about which the organization wishes to maintain data.
entity type
represents a collection of all entities that share common properties or characteristics. The entity type is described once in the database using meta
entity type
is a single occurrence of an entity type. For example; Mary, John, and Tom are 3 instances of the EMPLOYEE entity type.
entity instance
are the properties or characteristics of an entity. Each entity type has a set of attributes associated with it.
Attributes
association among one or more entities
relationship
is a meaningful association between or among entity types.
Relationship Type
is the association between specific entity instances.
Relationship Instance
was introduced by Peter Chen, who developed entity-relationship modeling and published his work in 1976. This notation was one of the pioneers in software and information system modeling and design.
Chen Notation
is used in Barker’s Notation, SSADM and Information Engineering. Crow’s Foot diagrams represent entities as boxes and relationships as lines between the boxes. Different shapes at the ends of these lines represent the cardinality of the relationship. Many preferred to use Crow’s Foot notation due to its simplicity.
Crow’s Feet Notation
Name the 3 types of entities
Entity / Strong Entity, Weak Entity, Associative Entity
This is an attribute that uniquely identifies a particular entity. The name of a key attribute is underlined in Chen Notation. In Crow’s Foot Notation, key attributes are also underlined. Aside from underline, it is also indicated with PF or FK depending on, if the key attribute is a primary key or foreign key (to be discussed in the succeeding module).
Key Attribute
This is an attribute of the weak entity that when combined with the key attribute of the parent entity, provides a unique identification for the weak entity. Chen notation underlines the discriminator with a dashed line. In Crow’s Foot Notation, the key attribute of the weak entity became composite, constituting the key attributes of the parent’s and the child’s.
Partial Key Attribute (discriminator)
An attribute that can have many actual values. For instance, a person may have several college degrees, or a household may have several different phones, each with its own number.
Multivalued Attribute
This is an attribute whose value is calculated (derived) from other attributes. The derived attribute may or may not be physically stored in the database. In the Chen notation, this attribute is represented by a dashed oval.
Derived Attribute (or computed attribute)
An attribute that can be further subdivided into smaller parts.
Composite Attribute
his is a relationship where the entity is strong and independent of other entities, and the Primary Key (PK) of child entry does not contain the PK component of the parent entity.
Strong Relationship
A relationship where child entity is existence-dependent on the parent entity, and PK of Child Entity contains PK component of Parent Entity. This relationship is represented by a double rhombus.
Weak Relationship / Identifying Relationship
A _______ of a Relationship is the number of relationship instances an entity can participate
Cardinality of Relationship
Each entity instance in one side of the relationship will have exactly one related entity instance on the other side of the relationship
One–to–One Relationship
