Data modelling

Data model

The term data model actually refers to two very different things: a description of data structure and the way data are organized using, for example, a database management system.

Data structure

A data model describes the structure of the data within a given domain and, by implication, the underlying structure of that domain itself. This means that a data model in fact specifies a dedicated 'grammar' for a dedicated artificial language for that domain.

A data model represents classes of entities (kinds of things) about which a company wishes to hold information, the attributes of that information, and relationships among those entities and (often implicit) relationships among those attributes. The model describes the organization of the data to some extent irrespective of how data might be represented in a computer system.

The entities represented by a data model can be the tangible entities, but models that include such concrete entity classes tend to change over time. Robust data models often identify abstractions of such entities. For example, a data model might include an entity class called "Person", representing all the people who interact with an organization. Such an abstract entity class is typically more appropriate than ones called "Vendor" or "Employee", which identify specific roles played by those people.

A proper conceptual data model describes the semantics of a subject area. It is a collection of assertions about the nature of the information that is used by one or more organizations. Proper entity classes are named with natural language words instead of technical jargon. Likewise, properly named relationships form concrete assertions about the subject area.

There are several versions of this. For example, a relationship called "is composed of" that is defined to operate on entity classes ORDER and LINE ITEM forms the following concrete assertion definition: Each ORDER "is composed of" one or more LINE ITEMS." A more rigorous approach is to force all relationship names to be prepositions, gerunds, or participals, with verbs being simply "must be" or "may be". This way, both cardinality and optionality can be handled semantically. This would mean that the relationship just cited would read in one direction, "Each ORDER may be composed of one or more LINE ITEMS" and in the other "Each LINE ITEM must be part of one and only one ORDER."

Note that this illustrates that often generic terms, such as 'is composed of', are defined to be limited in their use for a relationship between specific kinds of things, such as an order and an order line. This constraint is eliminated in the generic data modeling methodologies.

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