Relationship Constraints
Constraints on a relationship type limit the possible combinations of entities that may participate in its relationship set — determined by the Universe of Discourse (UoD), not chosen arbitrarily. Introduced in 2026-02-23-conceptual-database-design-and-the-er-model; see er-diagram-notation for the full symbol legend.
Cardinality ratio
Specifies the number of relationship instances an entity can participate in.
| Ratio | Meaning |
|---|---|
| 1:1 | Both entities can participate in only one relationship instance |
| 1:N | One entity can participate in many relationship instances (the “1” side, only one of the “N” side per instance) |
| N:1 | Same relationship, viewed from the other side |
| M:N | Both entities can participate in many different relationship instances |
Example: EMPLOYEE WORKSFOR DEPARTMENT — “each department can have any number of employees, but an employee can work for at most one department” is N:1 (N on the EMPLOYEE side, 1 on the DEPARTMENT side).
Participation constraint (existence dependency)
Indicates whether an entity’s existence depends on its relationship to another entity — i.e. whether every instance of the entity type must participate in at least one relationship instance.
- Total participation (double line in the diagram) — every entity instance must participate. E.g. “every employee must work for a department” — EMPLOYEE has total participation in
WORKSFOR. - Partial participation (single line) — an entity instance is not required to participate. E.g. “every employee can manage 0 or more departments” — EMPLOYEE has partial participation in
MANAGES.
Weak entities
An entity type with no key attribute of its own is a weak entity — it can only be identified uniquely by combining the primary key of its owner entity with its own partial key (underlined with a dotted line). The relationship linking a weak entity to its owner is the identifying relationship, and the weak entity always has total participation in it (it can’t exist without its owner). Both the weak entity box and the identifying relationship diamond are drawn with double lines. A weak entity can have two (or more) owner entity types, in which case the identifying relationship is n-ary rather than binary.