Content

1. Basics
2. Keys
3. E-R Diagram
   1. Attribute Types
   2. Weak Entity Set
   3. Role
   4. Specialization & Generalization
4. Relational Tables

Basics

• Entity
  • Attributes
• Relationship
• Constraints
  • Mapping cardinalities
    • One (->) v.s. > 1 (-)
  • Participation constraints
    • Total (=) v.s. Partial (-)

Keys

• Super key
  • A set of >= 1 attributes whose values uniquely determine each entity
• Candidate key
  • Minimal super key
  • Can be more than 1 candidate key
• Primary key (underline)
  • 1 of the candidate keys are selected as primary key
  • Cannot be NULL
  • Cannot be duplicated
  • Static
• Partial key (dashed underline)
  • Or discriminator
  • Distinguish among weak entities
• Foreign key
  • Referential constraint between two tables
  • Matches a candidate key of another table

E-R Diagram

Attribute Types

• Single attribute v.s. composite attribute
• Single-valued attribute v.s. multi-valued attribute (double ellipse)
• Derived attribute (dashed ellipse)

Weak Entity Set

Weak entity set (double rectangle) relates to an identifying entity set via total, many-to-one identifying relationship set (double diamond).

Primary key = primary key of identifying entity set + discriminator of weak entity set

Role

Entity sets of a relationship the same.

Specialization & Generalization

Subgroupings within an entity set.

Lower-level entity set inherites all attributes & relationship set participation of high-level entity set.

• Total or partial
• Disjoint or overlapping

Relational Tables