ENTITY RELATIONSHIP DIAGRAM - ERD ENTITY WHAT IS AN ENTITY? An entity is a real-world object, person, place, event, or concept. Examples: Student, Course, Lecturer, Booking, Payment Each entity has attributes (e.g., Name, ID, Address) Usually identified with a Primary Key (PK) ENTITY IN CHEN’S MODEL • Entities are rectangles with attributes as ovals. • Entity name is inside the rectangle. • Types: i. Strong Entity: Independent, has PK ii. Weak Entity: Depends on strong entity, no PK i. Strong entities • Exist independently of other types of entities • Has its own unique identifier • Represented with single-line rectangle ii. Weak entities • Dependent on a strong entities...cannot exist on its own • Does not have a unique identifier • Represented with double-line rectangle Types of Entity : Identifying relationship Links strong entities to weak entities Represented with double line diamond ENTITY, ATTRIBUTE AND RELATIONSHIP Student (ENTITY) Enrolled (RELATIONSHIP) Name (ATTRIBUTE) Course (ENTITY) ENTITY IN CROW’S FOOT NOTATION ENTITIES ARE RECTANGLES WITH ATTRIBUTES LISTED INSIDE. SHOWS PRIMARY KEYS (PK) AND FOREIGN KEYS (FK). RELATIONSHIPS ARE SHOWN USING LINES WITH CROW’S FOOT SYMBOLS. EXAMPLE - CROW’S FOOT NOTATION CHEN VS CROW’S FOOT - KEY DIFFERENCES CHEN: ATTRIBUTES AS OVALS | CROW’S FOOT: LISTED IN BOX CHEN: CONCEPTUAL FOCUS | CROW’S FOOT: TECHNICAL FOCUS CHEN: DIAMOND FOR RELATIONSHIP | CROW’S FOOT: LINES WITH SYMBOLS CROW’S FOOT IS COMMONLY USED IN INDUSTRY CHEN VS CROW’S FOOT - KEY DIFFERENCES Feature Chen’s Model Crow’s Foot Notation Entity Symbol Rectangle Rectangle Attribute Representation Ovals connected to entities Listed inside the entity box Relationship Symbol Diamonds Lines with crow’s foot symbols Weak Entity Visual Double rectangle No special symbol Focus Conceptual (academic learning) Practical (system implementation) Common Use Teaching, conceptual understanding Industry use, technical documentation SUMMARY • Chen’s Model: Great for learning basic ERD concepts • Crow’s Foot: Ideal for real-world system documentation • Both models represent entities and their relationships visually