A classification of the associations between entities [names]. In general there are very few relationships because the changing state of a relationship due to activities is usually represented by an attribute. For example:
- Jenny [entity] is going out with [relationship] Mark [entity]
- Jenny [entity] is married to [relationship] Mark [entity]
- Jenny [entity] has left [relationship] Mark [entity]
- Jenny [entity] has divorced [relationship] Mark [entity]
Can be better expressed as
Jenny [entity] is related to [relationship] Mark [entity]
with the states of the relationship being.......... going out with/or not; married to/or not; left/or not; and divorced or not. Within entity relationship models these are often represented by attributes of the combined Jenny/Mark combination.
As all relationships represent activity of some kind, they will be represented by a verb - has, is etc - and there will be rules about which activites are valid in which state. Thus Jenny cannot divorce Mark unless she has first married him. These rules are known from any cause/effect and function dependency diagrams.
In theory there may be a 'primal cause' [is created by] , the building of aggregates ['has'] , the simple change of state of a single entity ['is' eg Jenny is happy], the straightforward establishing of a relationship or breaking of that relationship ['is related to' as above].
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