Appendices

Appendix F. ER Diagram Symbols and Examples

The drawing vocabulary of Chapter 5: Chen and crow's-foot symbols, the constraint annotations, and worked examples on the university schema.

Chen notation

Element	Symbol	Meaning
(Strong) entity	Rectangle	Independently identified thing
Weak entity	Double rectangle	Identified only via its owner
Attribute	Oval	Property of an entity/relationship
Key attribute	Oval, name underlined	The identifier
Multivalued attribute	Double oval	A set of values
Derived attribute	Oval, dashed border	Computable (not stored)
Relationship	Diamond	Association among entities
Identifying relationship	Double diamond	Owner → weak entity
Cardinality (1:1, 1:N, M:N)	Labeled diamond edges	Pairing rule
Total participation	Double line	Every instance participates
Partial participation	Single line	Some instances may not
Specialization (ISA)	Triangle labeled ISA	Supertype → subtypes
Aggregation	Rectangle around a relationship	Relationship as an entity

Crow's-foot notation

End mark (at the many side)	Meaning
`—\|\|`	Exactly one
`—\|o`	Zero or one
`—<` (crow's foot)	Many (one or more)
`—o<`	Zero or many
Double/mandatory bar or solid crow's foot conventions	Total participation (tool-dependent)

Reading rule: walk each line aloud as entity-verb-entity with cardinality ("a department offers many courses").

The university ERD, crow's foot

 DEPARTMENT ──< OFFERS >── COURSE ──< RUNS >── COURSE_SECTION
     │                     (1 dept : N courses)   (1 course : N sections)
     │
     ├─o< MAJORS >── STUDENT ──< ENROLLS >── COURSE_SECTION
     │              (1 dept : N students)  (M:N, attribute: grade)
     │
 INSTRUCTOR ──< TEACHES >── COURSE_SECTION
              (1 instructor : N sections; partial on both sides)

The same facts, Chen fragments

     ┌────────┐  offers   ┌───────┐             (1,N)      (1,1)
     │DEPART- ├────< >───┤ COURSE│        DEPARTMENT ══ offers ══< COURSE
     │ MENT   │          └───────┘        (double line = total: every course
     └────────┘                             is offered by a department)

   STUDENT ══< enrolls >══ COURSE_SECTION      grade hangs on the diamond
            (partial participation on both sides)

Weak entity and specialization, drawn

  COURSE ══< has-section >══ ╔════════════╗        double rectangle:
                                 (weak)    ║ SECTION  ║        identified by
        owner          identifying        ╚════════════╝        (course_id,
                     (double diamond)     discriminator: section_no   section_no, term)

                    ┌────────┐
                    │ PERSON │
              ISA  /     │     \        specialization, disjoint or
                 ┌─────┐ ┌──────┐ ┌───────┐   overlapping; total or partial
                 │STU- │ │INSTR │ │ALUMNUS│
                 │DENT │ │UCTOR │ └───────┘
                 └─────┘ └──────┘

Constraint annotations to write beside any diagram

Cardinality per direction (1:1 / 1:N / M:N).
Participation per side (total/mandatory vs optional).
Weakness: owner + discriminator + identifying relationship.
Specialization: disjoint vs overlapping; total vs partial.
Relationship attributes (the grade, the quantity — never on the entity).
Business rules that will become constraints (the data-dictionary companion).

Diagram hygiene (the professional rules)

Name relationships with verb phrases; read the diagram aloud as sentences to review it.
Attributes in ovals only for small diagrams; real diagrams keep attributes in the data dictionary.
Mark keys explicitly — the diagram states identity.
Keep subtypes and weak entities marked, not implied.
The diagram communicates; the data dictionary specifies. Both travel together.