Data hoffer a 2013 entity type a collection of

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data” (Hoffer A 2013 )Entity Type: “A collection of entities that share common properties or characteristicsEntity Instance: “A single occurrence of an entity type”
A true data entity will have many possible instances Each of the instances will have something different that will differentiate it from other instances GUIDELINES TO RECOGNIZING ENTITIES
Strong Entity is an entity where all its instances exist independently of instances from other entity types Weak Entities is where the entity instances depend on instances from other entity type. STRONG VERSUS WEAK ENTITIES Employeeis a strong entity type Employee card is a weak entity type
Strong Entity : Rectangle with an entity type name inside it ENTITY TYPES NOTATION IN ERD <Entity Type Name> Weak Entity : Double line rectangle with an entity type name inside it <Entity Type Name>
Is a characteristic that describes an entity Each entity instance within an entity type will have the same attributes, which are the attributes of it entity type The attribute is a holder to which we associate values (data) ATTRIBUTES
ATTRIBUTES NOTATION IN ERD <Entity Type Name> Attribute 1 Attribute2 Attribute 3 Add the attributes inside their entity type rectangle
SIMPLE (ATOMIC) VS COMPOSITE ATTRIBUTES Simple Attribute Composite Attribute Difference Can not be broken to smaller meaningful components Can be broken to smaller meaningful components example Age Address, Full Name
SINGLE VALUE VS MULTIVALUED ATTRIBUTES Single-Valued Multivalued Difference Has only one value (data) associated to the attribute Has more than one value (data) associated to the attribute example Address Employee skills
STORED VS DERIVED ATTRIBUTES Stored Attribute Derived Difference Is the origin of the data. Can not recreate its associated data Can be recreated from other attribute values example Date Employed Years Employed
Is an attribute or a combination of more that one attributes whose value will uniquely identify each entity instance within an entity type Each entity must have an identifier Student Id, Social Insurance Number are examples of identifier attributes IDENTIFIER ATTRIBUTE
DIFFERENT ATTRIBUTE TYPES NOTATION IN ERD EMPLOYEE
DEFINITIONS Attribute: “A property or Characteristic of an entity or relationship type that is of interest to the organizationRequired Attribute: “An attribute that must have a value for every entity instance with which it is associated” Optional Attribute: “An attribute that may not have a value for every entity instance with which it associates
DEFINITIONS Composite Attribute: “Anattribute that has meaningful component parts (attributes)” Simple (or Atomic) Attribute: “An attribute that cannot be broken down into smaller components Multivalued Attribute: “An attribute that may take on more than one value for a given entity (or relationship) instance
DEFINITIONS Derived Attribute: “An attribute whose values can be calculated from related attribute valuesIdentifier:

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