Slide 16.8: Desirable properties

Desired Properties

When extending SQL, the following aspects seem to be important:

Values have to be distinguished from objects: object identities.
Objects may have a complex structure which demands special navigational support.
Some properties of objects may be defined by inheritance.
Because of set-valued attributes and class extensions, sets play a more prominent role.
Creation of objects.

Elementary Access to Objects
Every object has an identity independent of its value.

Asks for the identities of those vehicles whose attribute Model has the value Tipo:
SELECT f FROM f IN vehicle WHERE model='Tipo';
where f are object identities. Identities are not of interest in general. They are generated by system rather than assigned as names by user.

Asks for all attribute values of vehicles with the model value Tipo:
SELECT * FROM vehicle WHERE model='Tipo';

Access to Complex Objects
Complex objects which are structured in a complex way originate from atomic or already constructed objects by applying certain constructors.

Path expressions are allowed in the select-clause:

SELECT president.salary FROM company WHERE headoffice.location='Rome';