EER schemas can be expressed in a diagrammatic form called
*EER Diagram*. Entity-sets, relationship-sets, and
attributes are represented graphically by rectangles, diamonds, and
ellipses, respectively. Every vertex is labeled by the name of the
object-set or attribute it represents; entity and relationship vertices
must be uniquely identified by their labels globally, while attribute
vertices must be uniquely identified by their labels only locally, with
respect to their object-set.

Edges in an EER diagram are directed and represent the interaction of the various object-sets and attributes. Thus, there are edges:

- directed from vertices representing entity-sets or relationship-sets to vertices representing attributes;
- directed from vertices representing relationship-sets to vertices representing entity-sets or other relationship-sets;
- directed from vertices representing weak entity-sets
to vertices representing other entity-sets, labeled
*ID*; - directed from vertices
representing entity-sets to vertices representing other entity-sets,
labeled
`HAS`or`ALWAYS_HAS`;

**Figure 1:**An Extended Entity-Relationship Diagram Example

- directed from vertices
representing specialization entity-sets
to vertices representing generic entity-sets,
labeled
*ISA*.

If entity-set `E` has an association-cardinality
of *one* (respectively *many*) with respect to
relationship-set `R`,
then the edge connecting the vertices representing `E` and `R`
is associated with label *1* (respectively *M*).
Mandatory involvement of entity-sets in relationship-sets is represented
graphically in an EER diagram by *double-line* edges instead of
the regular edges
representing the non-mandatory (optional) involvements.

Figure 1 contains an example of
an EER diagram consisting of the following main elements:
`PERSON`, `AUTHOR`, `PROJECT`,
`REFERENCE`, `PUBLICATION`, and `SPONSOR`
are entity-sets, relationship-set
`AUTHORED` represents the association of authors with references,
relationship-set `ASSIGNED` represents
the assignment of projects to persons, `HAS`
represents the project-sub-project relation, and
`SPONSORS` represents the sponsoring of projects.
`PERSON` represents a generalization of entity-set `AUTHOR`.
The association cardinalities of relationship-set
`ASSIGNED` represent the restriction of
a project being assigned to at most one person.