Entity-Relationship Model In DBMS

He also did the same analysis for Chinese characters.


Identifiers or keys: One or more attributes that uniquely identify each entity instance
Simple attribute: Attribute that cannot be subdivided
Most attributes are this way
Single-valued attribute: Attribute that has only a single value at a time
Most attributes are this way
Multivalued attributes: Attributes that have many values
Generally, to be avoided unless necessary for performance or to match common app accesses.
Slides 2 and 3A Multivalued Attribute in an Entity (CAR_COLOR)
These require creating:
Several new attributes, one for each
component of the original multivalued attribute
Derived attribute: Attribute whose value is calculated from other attributes
Derived using a formula or an algorithm
Slide 4- Depiction of a Derived Attribute
Slide 5- Advantages and Disadvantages of Storing Derived Attributes

Entities: Strong vs. Weak

Entities can either exist on their own or they can only exist when associated with some other entity type.

Strong entities – A strong entity can be uniquely identified by its own attributes. – Therefore, the entity’s existence does not depend on any other entity – e.g., a Dormitory can be uniquely identified by its name and location.
– e.g., a US Bank is uniquely identified by its bank number.
Weak entities
Thus, you must add attributes to the weak entity to uniquely identify it.
This means you must extend the weak entity’s primary key to include one or more attributes from the parent entity as a foreign key.
g., a Room in a Dormitory needs the Dormitory information as part of its identity.
g., an Account may be identified by an Account Number, but it is meaningless without being associated with a Bank.
Alternatively, you could add a surrogatekey to the weak entity.
These keys are not related to the entity’s real attributes, such as an AUTO INCREMENTor other generated value. Adding a surrogate key would turn it into a strong entity


An association between entities, typically meaningful in both directions

Participants: Entities that participate in a relationship
Connectivity: Describes the relationship classification
Cardinality: (optional) Expresses the minimum and maximum number of entity occurrences associated with one occurrence of a related entity
Slide 6- Connectivity and Cardinality in an ERD
Existence Dependence/Independence
Some Entities exists in the database only when associated with another entity occurrence
Others exist on their own without dependence on other entities existence
Relationship strength
A weak or non-identifyingrelationship exists between two entities when the primary key of one of the related entities does not contain a primary key component of the other related entities.
Slide 7- A Weak (Non-Identifying) Relationship between COURSE and CLASS
A strong or identifyingrelationship is when the primary key of the related entity contains the primary key of the “parent”.