Single Value Constraints
An entity (or object) may have at most one value for a given
attribute or relationship.
Person: name, social-security number
How do we do this in ODL?
In E/R, every attribute has at most one value.
Arrows tell us about multiplicity of relations.
If we have a single-valued constraint, we can either:
1. Require that the value exist (see referential integrity shortly)