is 325 notes for wednesday september 4, 2013
DESCRIPTION
IS 325 Notes for Wednesday September 4, 2013. Syllabus Change. I eliminated quizzes I increased the points allocated to homework assignments. Entity-Relationship Modeling/Diagrams. Entity-relationship (ER) modeling - conceptual database modeling technique - PowerPoint PPT PresentationTRANSCRIPT
IS 325
Notes for
Wednesday
September 4, 2013
Syllabus Change
• I eliminated quizzes
• I increased the points allocated to homework assignments
Entity-Relationship Modeling/Diagrams
• Entity-relationship (ER) modeling - conceptual database modeling technique Enables the structuring and organizing of the
requirements collection process Provides a way to graphically represent the
requirements
• ER diagram (ERD) - the result of ER modeling Serves as a blueprint for the database
ENTITIES
• Entities - constructs that represent what the database keeps track of… something of specific interestThe basic building blocks of an ER diagramRepresent various real world notions, such as
people, places, objects, events, items, and other concepts (think “nouns”)
Within an ERD each entity must have a different name and different semantic meaning
ENTITIES
Two entities
ENTITIES
• Entity instances (entity members) - occurrences of an entityEntities themselves are depicted in the ER
diagrams while entity instances are notEntity instances are eventually recorded in the
database that is created based on the ER diagramEach entity instance becomes a row in the
database
ATTRIBUTES
• Attribute - a characteristic of an entityRepresents the details that will be recorded for each
entity instanceWithin one entity, each attribute must have a
different name and different semantic meaning
• Unique Attribute/Set of Attributes – an attribute or set of attributes that have a value(s) is different for each entity instanceEvery regular entity must have at least one unique
attribute or a set of attributes (think George Foreman)
ATTRIBUTES
An entity with attributes
RELATIONSHIPS
• Relationship – meaningful or naturally occurring associations between entitiesWithin an ER diagram, each entity must be related
to at least one other entity via a relationship
RELATIONSHIPS
• Cardinality constraints - depict how many instances of one entity can be associated with instances of another entityMaximum cardinality
One (represented by a straight bar I (Roman Numeral 1))
Many (represented by a crow’s foot symbol)
Minimum cardinality (participation)Zero or Optional (represented by a circular symbol: 0
(Roman Numeral 0))
Mandatory (represented by a straight bar I (roman Numeral 1))
RELATIONSHIPS
A relationship between two entities
RELATIONSHIPS
Four possible cardinality constraints
RELATIONSHIPS
Several possible versions of the relationship "ReportsTo"
RELATIONSHIPS
• Types of Relationships (maximum cardinality-wise)One-to-one relationship (1:1)One-to-many relationship (1:M)Many-to-many relationship (M:N)
RELATIONSHIPS
Three types of relationships (maximum cardinality-wise)
RELATIONSHIPS
1:M Relationship
M:N Relationship
1:1 Relationship
RELATIONSHIPS
• Relationship instances - occurrences of a relationshipOccur when an instance of one entity is naturally
or meaningfully associated to an instance of another entity via a relationship
Relationship themselves are depicted in the ER diagrams while relationship instances are not
Relationship instances are eventually recorded in the database that is created based on the ER diagram
RELATIONSHIPS
A relationship and its instances
RELATIONSHIPS
• Relationship attributes In some cases M:N relationships can actually have
attributes of their own
RELATIONSHIPS
A M:N relationship with an attribute
RELATIONSHIPS
A 1:M relationship with and without an attribute
ER diagram example: ZAGI Retail Company Sales Department Database
ATTRIBUTES
• Composite attribute – attribute that is composed of several attributes (an amalgam or composite)Not an additional attribute of an entityIts purpose is to indicate a situation in which a
collection of attributes has an additional meaning, besides the individual meanings of each attribute
ATTRIBUTES
An entity with a composite attribute
ATTRIBUTES
Another entity with a composite attribute
ATTRIBUTES
Composite attributes sharing components
ATTRIBUTES
• Composite unique attribute – attribute that is composed of several attributes and whose value is different for each entity instance
ATTRIBUTES
An entity with a unique composite attribute
ATTRIBUTES
• Multiple unique single attributes or minimal sets of attributes (candidate keys) - when an entity has more than one unique attribute each unique attribute is also called a candidate key
ATTRIBUTES
An entity with multiple unique attributes (candidate keys)
ATTRIBUTES
An entity with a candidate key and composite key (which does not satisfy the requirements of of a candidate key… I differ with the text authors)
ATTRIBUTES
• Multivalued attribute - attribute for which instances of an entity can have multiple values for the same attribute
ATTRIBUTES
A multivalued attribute
ATTRIBUTES
A scenario that does not use multivalued attributes
ATTRIBUTES
• Derived attribute - attribute whose values are calculated and not permanently stored in a database
ATTRIBUTES
A derived attribute example
ATTRIBUTES
Another derived attribute example
ATTRIBUTES
• Optional attribute - attribute that is allowed to not have a value
ATTRIBUTES
An optional attribute example
ATTRIBUTES
EXAMPLE: An entity with various types of attributes
RELATIONSHIPS
• Exact minimum and maximum cardinality in relationships In some cases the exact minimum and/or maximum
cardinality in relationships is known in advanceExact minimum/and or maximum cardinalities can
be depicted in ER diagrams
RELATIONSHIPS
A relationship with specific minimum and maximum cardinalities
RELATIONSHIPS
RELATIONSHIPS
• Degree of a relationship - reflects how many entities are involved in the relationship
• Binary relationship - relationship between two entities (degree 2 relationship)
• Unary relationship (recursive relationship) - occurs when an entity is involved in a relationship with itself (degree 1 relationship)
RELATIONSHIPS
Unary relationship examples
RELATIONSHIPS
• Relationship roles - additional syntax that can be used in ER diagrams at the discretion of a data modeler to clarify the role of each entity in a relationship
RELATIONSHIPS
Unary relationships with role names
RELATIONSHIPS
A binary relationship with role names
RELATIONSHIPS
• Multiple relationships between same entitiesSame entities in an ER diagram can be related via
more than one relationship
RELATIONSHIPS
Multiple relationships between the same entities
Chalk Talk And Examples
• Let’s practice