www.entitymodelling.org - entity modelling introduced from first principles - relational database design theory and practice - dependent type theory
In computer science the term ‘relational’ applies to a particular style of data table; its use in this sense we describe here except that we describe it in a general setting by considering ‘relational’ as applying to a certain style of message system. Instead of saying ‘columns of a table’ I shall say ‘attributes within a message structure’; this choice of terminology is made so as to ensure both that concepts are presented in their full generality and also that software tools that support the concepts have the widest possible applicability.
So what is meant by relational when applied to a message structure? We ask this question in the context of entity modelling; we assume therefore an entity model and a system of messages so that each message communicates what we know of an entity. In this context a message is relational if and only if it communicates only attributes of a single subject entity and if it does not rely on context; in particular (a) it does nor rely on context to determine the identity of the subject entity and (b) it does not rely on context for the interpretation of referential attributes.
A message system which has full coverage of an entity model is called a relational database schema. Relational database design as an after step of entity modelling is to design a minimum such message system i.e one from which if any attribute of any message is removed from the system the the system no longer has coverage of the entity model.
Earlier, in display (9) of section attributes we gave examples of messages conveying information regarding chemical compounds aspirin, common salt and water, their aliases, their molar masses and their chemical formulae. In that example there was one messsage per each of these three example compounds and each message included attributes from subordinate entities of the compound entity i.e from its aliass and its occuring elements as well as including attributes of the compound itself. The same information can be conveyed relationally in the fourteen messages shown below in (10). Both (9) and (10) convey the same information.
This set of messages (10) is relational because each message gives information about a single entity type. The information can be conveyed in three tables, one per subject entity type, as shown in figure 7.
|aliased name||compound name|
|sodium chloride||common salt|