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Additional problem appears when the equality is between two IRIs of two different IRI classes. Even if both of them are bijections, the compiler does not know if these IRI classes behave identically on the intersection of their domains. To let the optimizer know this fact, one IRI class can be explicitly declared as a subclass of another:

make oplsioc:user_iri subclass of oplsioc:grantee_iri .
make oplsioc:group_iri subclass of oplsioc:grantee_iri .

The SPARQL compiler can not check the validity of a subclass declaration. The developer should carefully test functions to ensure that transformations are really subclasses, as well as to ensure that functions of an IRI class declarations are really inverse to each other.

When declaring that a table's primary key is converted into a IRI according to one IRI class, one usually declares that all foreign keys referring to this class also get converted into an IRI as per this same class, or subclass of same class.

Subclasses can be declared for literal classes as well as for IRI classes, but this case is rare. The reason is that most of literals are made by identity literal classes that are disjoint to each other even if values may be equal in SQL sense, such as "2" of type xsd:integer and "2.0" of type xsd:double .