Database management systems

1
Database management systems

• The Entity Relationship Model

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The Steps of Database Design

• Requirements Analysis
• Conceptual Design
• Logical Design
• Physical Design

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The Entity Relationship Model

• This data model allows us to describe data from a
  real-world enterprise in terms of objects and
  their relationships
• Create a miniworld Elmasri RA

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Entities

• Entity a distinguishable object
• Entity Set a collection of similar entities
• Entity Type describes the intension for a set
  of entities that share the same attributes
• Attributes components of an entity

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Relationships

• Relationship a distinguishable association
  among entities
• Relationship Set a collection of associations
• Relationship Type describes a set of meaningful
  associations among entity types
• Attributes components of a relationship
• Degrees
• Unary - recursive
• Binary
• Ternary
• N-ary

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Attributes

• Properties of an entity or relationship
• Consist of a
• Description
• Domain
• Classified as
• Simple atomic values with independent existence
• Composite multiple components
• Single-valued
• Multi-valued atypical in relational model
• Derived determined from values in other
  attributes

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Keys

• Candidate key a minimal set of attributes that
  uniquely identifies each occurrence of an entity
  type
• Primary key a selected candidate key for an
  entity
• Composite key indicates that two or more
  attributes exist in the key

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Weak Entities (vs Strong Entities)

• Attributes do not allow uniqueness (i.e., no key
  within the entity itself
• Identifiable only in conjunction with the primary
  key of another strong entity in a 11 or 1N
  relationship
• Synonyms Child, Dependent, Subordinate
• Must have total participation on the weak entity
  side

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Structural Constraints

• Key Constraints (multiplicity)
• The number of possible occurrences of an entity
  type associated with another entity type
• Cardinality ratio
• 11
• 1 (1N) 1..0 1..
• (NN)
• Participation Constraints
• Mandatory (Total) every entity must participate
• Optional (Partial) 0 to total participation

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Alternate diagrammatic methods
11
Diagrammatic translations
Manages gt
0..1
1..1
Manages
Staff
Branch
1
0..1
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Class Hierarchies

• Class Hierarchies
• Entities can be placed in subclasses
• Inheritance by attributes of the subclass
  subclass ISA class
• Queries on a class must consider the subclass
  entities as well
• There can be multiple ISA nodes

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Class Hierarchies

• Can be viewed in 2 ways
• Class specialized into subclass
• Subclasses generalized by class
• Constraints on ISA hierarchies
• Overlap
• Covering
• Why have them?
• Specific descriptive attributes
• To easily identify all participants

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Aggregation

• Differs from relationships, which associate
  entities
• Useful when a relationship set participates in
  another relationship set