Module 4

• E-R diagrams (ERD)

  • represent logical structure of a database
  • shows the relationships of entity sets stored in a database
  • entity is an object, a component of data
• Entity Set

  • collection of similar entities
  • can have attributes that define their properties
• Meaning of Rectangle

  Entity Set (Strong)
• Meaning of Symbol: Double Rectangle


  Entity Set (Weak)
• Meaning of Symbol: Elipse

  Attribute
• Meaning of Symbol: Double Ellipse

  Multi-valued Attributes
• Meaning of Symbol: Ellipse with a line inside

  Key attribute
• Meaning of Symbol: Ellipse joined with other ellipses

  Composite Attributes
• Meaning of Symbol: Diamond

  Relationship Set
• Meaning of Symbol: Double Diamond

  Identifying Relationship Type
• Meaning of Symbol: Single Line

  Partial Participation
• Meaning of Symbol: Double Lines

  Total participation
• Meaning of Symbol: Triangle (ISA)


  Special or Generalization
• Seven Advantages of E-R Model (1) Straightforward relation presentation (2) Mapping with relational model (3) Communication tool (4) Design tool (5) Easy conversion to other models (6) Graphical representation (7) Easy to modify
• Straightforward relation representation


  database model’s relation representation using the E-R diagram is relatively more straightforward than other models
• Mapping with relational model

  • easily mapped onto the relational model
  • E-R diagrams used in E-R model can easily be transformed into relational tables 
  • entities & attributes of E-R model can easily be transformed into relations (tables) and columns (fields) in a relational model
• Communication tool

  • straightforward to understand with minimum required training
  • database designer can use E-R model to communicate the design to end-user
• Design tool


  be used as a design plan by the database developer to implement a data model in specific database management software
• Easy conversion to other models

  E-R diagrams can be easily converted to a network or hierarchical data model
• Graphical representation

  E-R model provides a graphical & diagrammatic representation of various entities, their attributes, and relationships between entities
• Easy to modify

  modifications to E-R diagram at a later stage is relatively easier than in other models
• Limitation of E-R Model cannot express relationships between relationship/s (sets)
• Types of Entity Sets

  1. Strong Entity Sets 2. Weak Entity Sets
• Determine what is the type of entity sets and what is the name of the symbol and the type of attribute of Reg_No Answer: Weak Entity Set, Ellipse with a line inside - Key attribute

  
• Determine what is the type of entity sets and what is the name of the symbol of TABLE

  Answer: Weak Entity Set, Double Rectangle, Attribute

  
• Weak Entity Sets

  Entity sets having no key attributes
• Strong Entity Sets

  Entity sets having any key attributes