Key Learnings

By azilaainul.cf | Updated: Aug. 24, 2024, 4:46 p.m.

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Data Modelling
DR AINUL AZILA CHE FAUZI
Understanding the Entity Relationship Diagrams
01
Notation Basics
02
Understanding relationships
Basic Symbols
Terminology
Entity Types
A collection of entities 
(noun -> persons, places, events, or things) of interest represented by a rectangle in an ERD

Relationship
A named (verb -> has, teaches, registers) association among entity types, usually two-way or bidirectional

Terminology
Attribute
A property of an entity type or relationship.  Each attribute has a data type that defines the kind of values and permissible operations on the attribute

Reading ERD
Use active verb rather than passive verb eg “course has offerings” rather than “offering is given for a course”

Reading ERD
A constraint on the number of entities that participate in a relationship.  Has min and max cardinality for both directions of a relationship

Cardinality Notation
Understanding Relationships
Identification dependency
M-N relationships with attributes
Self identifying relationships
M-way relationships
Equivalence between
M-N and 1-M 
relationships
Identification Dependency
Strong Entity Type:
has its own primary key (PK) eg. STUDENT has matric#

does not have its own PK but borrows part (or all) PK from other strong entity types eg PK for STATE is (StateID, CountryID), PK for DEPENDENT is (DependentIC, StaffID) used in UiTM Panel Clinic System

Weak Entity Type:
Identifying Relationship

The relationship(s) that provides components of the PK for weak entity type eg contains in BUILDING and ROOM or holds in COUNTRY and STATE

Involves a weak entity type and one or more identifying relationshipsBecause of the existence dependency and the PK borrowing, the min and max cardinalities of a weak entity are always 1
Identifying Dependency

Identification Dependency
M-N Relationships with Attributes
Relationships can have attributes especially M:N relationships eg EnrGradeEnrGrade must be associated with the combination of STUDENT enrolls in OFFERING
Self-Referencing Relationships
Also called unary or reflexive relationship, involves connections among members of the same set/entity type
Associative Entity Types for M-way (Multiway) Relationships
What is 
Generalization hierarchies??
Is a collection of entity types arranged in a hierarchical structure to show similarity in attributes, hence reduced null values in Employee

Each subtype (child entity type) contains a subset of entities of its supertype (parent entity type)

Each subtype entity IS A supertype entity, therefore it is not shown on the diagram, replaced by thick line with arrow up or down

Generalization Hierarchies
Subtypes inherit attributes of supertypes (direct-EmpNo and indirect- EmpName, EmpHireDate)

Example:
Employee (EmpNo, EmpNameEmpHireDate)
SalaryEmp (EmpNo, EmpSalary) *
HourlyEmp (EmpNo, EmpRate) *
* EmpNo is both PK and FK

Inheritance
Disjointness constraint means that subtypes in a generalization hierarchy do not have any entities in common eg Security cannot be both a Stock and a Bond

Completeness constraint means that every entity of a supertype must be an entity in one of the subtypes eg every Security must be either a Stock or a Bond
Some generalization hierarchies lack both one or both constraints (no symbol D,C)
Generalization Constraints
Generalization Constraints
Eg Every Security entity must be either a Stock entity or a Bond entity
Ensure that ERD notation is correctly used
Similar to syntax rules for a computer language
Completeness rules: no missing specifications
Diagram 
Rules
Consistency rules: no conflicts among specifications
Supported by the ER Assistant (software)
Completeness rules: no missing specifications
Primary Key Rule Issue

For some weak entities, the PK rule is subtle
Weak entity with only one 1-M identifying relationship
Weak entity must have a local key to augment the borrowed PK from the parent entity type
Violation of PK rule if local key is missing
Entity Name Rule:
Attribute Name Rule:
Inherited Attribute Rule:
Naming Consistency Rules
entity type names must be unique

attribute names must be unique within each entity type and relationship

attribute names in a subtype do not match inherited (direct or indirect) attribute names.

The End

Key Learnings

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