Entity Modelling

www.entitymodelling.org - entity modelling introduced from first principles - relational database design theory and practice - dependent type theory


Previously we gave this example model of an atomic nucleus:

This model can be expressed in a different way using the generalisation nucleon of the individual terms proton and neutron by stating: an atomic nucleus is composed of one or more nucleons; every nucleon is either a proton or a neutron. In an entity model diagram the generalisation type is shown as a box containing the specific types; so the model may be expressed thus:

Be aware that there is a minor difference between this way of expressing the situation and the first form: when expressed in this latter form the model is consistent with a nucleus with one neutron and zero protons - physics teaches us that such nuclei will not occur in the real world.

The same generalisation can be used to express that both protons and neutrons are combinations of quarks: