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Conceptual Database Design: The Entity Relationship (ER) Model, Ch 4…
Conceptual Database Design: The Entity Relationship (ER) Model
Overview
Mini-world: a part of the real world that we are interested in modelling
Modeling process
Requirements collection/analysis
Conceptual design
Logical design
Physical design
Importance of conceptual design
Fundamental
Involves users
Provides documentations for the database
Independent of DBMS
ER model and constructors (building blocks)
Entity classes
Entity: a data object that exists and is distinguishable from other objects
Entity type: a description of the attributes that a set of entities has in common
Entity set: a collection of entities of the same type
Entity class: combination of entity set + entity type
Attributes: property of an entity, minimal
Assumption: Each entity will have a unique combination of
attribute values.
Identifier: one or mare attributes whose values uniquely determine each entity, it's minimal
Primary identifier: one of the identifier
Unique, mandatory, time-invariant
Null value: @
Single value vs. Multi-value
Store vs. Derived
Cardinality: a set refers to the number of elements in that set
Entity, constraint bounds the upper and/or the lower limits of the cardinality
Attributes, restricts the number of values an attribute can take on
Relationship classes
Interaction relationship, or relationship: an association between (or among) entities
Relationship type: an
ordered
list of participating entity classes defined as. ex. Customers "Buys" Produtcs
Relationship set: a specific set of associated entity instances based on relationship type. ex. (Customer1, Product1), (Customer1, Product2)
Relationship class = relationship type + relationship set
Degree: number of entity classes (participants)
involved in a relationship
Unary
Binary
Ternary
Higher order relationship
Cardinality: in the context of relationships, refers to the number of members of another entity class, that a given entity (or entity combination) can be associated with.
be at a point in time
across all relevant
space
Weak entity classes: associated data, to provide more data about the interaction between entities
inherits identifying attributes from another entity class (called the
owning
or
strong
entity class) to uniquely identify its own entities
Characteristics
Independent Identification
partial identifier
if the max cardinality > 1 in its identifying relationship
Superclasses and subclasses
a database can be modelled as
a collection of entity classes
relationships among entity classes
Recommendations and review
Ch 4 Entity Relationship (ER) Modeling