Introduction to database and data communication, Exams of Data Communication Systems and Computer Networks

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CHAPTER 3-

DATA MODELING

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In this chapter, you will learn:

• (^) What a conceptual model is and what its purpose is
• (^) The difference between internal and external models
• (^) How internal and external models serve the database design process
• (^) How relationships between entities are defined and refined, and how such relationships are incorporated into the database design process
• (^) How ERD components affect database design and implementation
• (^) How to interpret the modeling symbols for the four most popular E-R modeling tools
• (^) That real-world database design often requires you to reconcile conflicting goals

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Data Models: Degrees of Data Abstraction Figure 3.

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Degrees of Abstraction

• (^) Conceptual
  • (^) Global view of data
  • (^) Basis for identification and description of main data items
  • (^) ERD used to represent conceptual data model
  • (^) Hardware and software independent
• (^) Internal
  • (^) Representation of database as seen by DBMS
  • (^) Adapts conceptual model to specific DBMS
  • (^) Software dependent

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The Entity Relationship (E-R) Model

• (^) Represents conceptual view
• (^) Main Components
  • (^) Entities
    • Corresponds to entire table, not row
    • Represented by rectangle
  • (^) Attributes
  • (^) Relationships

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Attributes

• (^) Characteristics of entities
• (^) Domain is set of possible values
• (^) Primary keys underlined Figure 3.

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Relationships

• (^) Association between entities
• (^) Connected entities are called participants
• (^) Operate in both directions
• (^) Connectivity describes relationship classification
  • (^) 1:1, 1:M, M:N
• (^) Cardinality
  • (^) Expresses number of entity occurrences associated with one occurrence of related entity

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Relationship Strength

• (^) Existence dependence
  • (^) Entity’s existence depends on existence of related entities
  • (^) Existence-independent entities can exist apart from related entities
  • (^) EMPLOYEE claims DEPENDENT
• (^) Weak (non-identifying)
  • (^) One entity is existence-independent on another
  • (^) PK of related entity doesn’t contain PK component of parent entity
• Strong (identifying)
  • (^) One entity is existence-dependent on another
  • (^) PK of related entity contains PK component of parent entity

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Weak Entity

• (^) Existence-dependent on another entity
• (^) Has primary key that is partially or totally derived from parent entity Figure 3.

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Relationship Degree

• (^) Indicates number of associated entities
• (^) Unary
  • (^) Single entity
  • (^) Recursive
  • (^) Exists between occurrences of same entity set
• (^) Binary
  • (^) Two entities associated
• (^) Ternary
  • (^) Three entities associated

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Composite Entities

• (^) Used to ‘bridge’ between M:N relationships
• (^) Bridge entities composed of primary keys of each entity needing connection Figure 3.

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Composite Entities (con’t.) Figure 3.

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Generalization Hierarchy with Overlapping Subtypes Figure 3.

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Comparison of E-R Modeling Symbols

• (^) Alternate styles developed to enable easier use of CASE tools
• (^) Chen
  • (^) Moved conceptual design into practical database design arena
• (^) Crow’s Foot
  • (^) Cannot detail all cardinalities
• (^) Rein
  • (^) Similar to Crow’s Foot
  • (^) Operates at higher level of abstraction
• (^) IDEF1X
  • (^) Derivative of ICAM studies in the late 1970’s
  • (^) Uses fewer symbols