ER Diagram Notation

Reference legend for the symbols used across every ER/EER diagram in this course — introduced across 2026-02-23-conceptual-database-design-and-the-er-model and 2026-03-02-weak-entities-the-eer-model-and-design-choices.

Core symbols

Symbol	Meaning
Rectangle	Entity type
Double-line rectangle	Weak entity type (no key attribute of its own)
Oval	Attribute
Oval, name underlined	Key attribute
Oval, name underlined with a dotted line	Partial key attribute (weak entity’s own distinguishing attribute)
Double-line oval	Multivalued attribute (can hold more than one value)
Dashed-line oval	Derived attribute (computed from another stored attribute, e.g. Age from BirthDate)
Diamond	Relationship type
Double-line diamond	Identifying relationship type (connects a weak entity to its owner entity)

• Composite attribute: an attribute with its own sub-attributes attached (e.g. Name splitting into FirstName/MiddleName/LastName) — the parent attribute oval isn’t underlined/dashed/doubled itself unless it’s also a key/derived/multivalued attribute.
• Value sets (the domain of legal values for an attribute, e.g. “integers 21-65”) are never shown on the diagram itself.

Relationship constraints

See relationship-constraints for cardinality ratio (1:1, 1:N, M:N) and participation (total/partial) notation.

Subclasses / superclasses (EER)

Symbol	Meaning
⊂ between two entity types	The entity type on the narrow side is a subclass of the one on the wide side
A circle joining several subclasses to one superclass	Several entity types are all subclasses of the same superclass
d inside that circle	Disjoint specialization — an entity instance can belong to at most one of the subclasses
o inside that circle	Overlapping specialization — an entity instance can belong to more than one subclass at once
Single line from superclass to the subclass circle	Partial specialization — not every superclass instance needs to belong to a subclass
Double line from superclass to the subclass circle	Total specialization — every superclass instance must belong to at least one subclass

Naming conventions (INFS1200/7900 style guide)

Course-specific naming standard for entities/relationships/attributes on ER and EER diagrams (distinct from — and in addition to — the mandatory notation above):

• Entity names: capitalised, no spaces, ideally one word (e.g. STUDENT, POLICEOFFICER — not PoliceOfficer).
• Relationship names: capitalised, no spaces, ideally one word, preferably a verb (e.g. CREATES, ACTSIN — not LeadActor).
• Attribute names: UpperCamelCase — first letter of each word capitalised, no spaces, acronym letters stay capitalised (e.g. ComputerIP, DateOfBirth — not Date of Birth).

How diagrams are drawn in these notes

Diagrams in this course’s notes are drawn as Mermaid flowchart graphs (renders in both PDF and HTML), using shapes chosen to match the Chen notation above as closely as Mermaid’s flowchart shapes allow:

Mermaid shape	Used for
[Entity] (rectangle)	Entity type
[[WeakEntity]] (subroutine, double vertical bars)	Weak entity type
{Relationship} (diamond/rhombus)	Relationship type
([Attribute]) (stadium)	Attribute
((d)) / ((o)) (small circle)	Disjoint/overlapping specialisation marker

Since Mermaid can’t reliably underline/dash node text across renderers, attribute qualifiers are written as a plain-text suffix instead: (key), (partial key), (derived), (multi). Composite attributes are drawn as a parent attribute node connected to its component attribute nodes. Cardinality is a text label on the edge ("1", "N", "M"); total participation/identifying relationships use a thick edge (===/==>), partial participation uses a thin edge (---/-->).

Any diagram with more than ~6-8 nodes needs an explicit %%| fig-width: cell option (in inches) or it runs off the right edge of the PDF page - mermaid sizes itself from the diagram’s own SVG bounding box, and there’s no project-wide default that reaches it (a document/project fig-width only applies to executed-code-cell figures, e.g. matplotlib). 5.5 fits this vault’s PDF page width (scrartcl, DIV=11, letter) comfortably:

```{mermaid}
%%| fig-width: 5.5
flowchart TD
    ...
```