Describing a Database

The reason that databases are incorrectly designed has a lot to do with the Data Description Languages (DDLs). Data description in COBOL, SQL, and XML all omit some crucial elements.

A database description names and identifies collections of similar things and what it is that you want to know about them. Ontologically, for a thing to exist in a database, something about that thing must be explicitly stored; that is, at least one property of that thing must be stored in the database. It could be as simple as an arbitrarily assigned identifier. For example, each person in a collection of people might be assigned a unique identifying number. In Mealy's terms, an example would be: 'this person (object) has an identifier (attribute) with a (value) of 3097.' Databases consist of primary objects, the sets of objects that are the reason for the database's existence; temporal objects, the sets of data about the primary objects that change over time; the data domains, the sets of values (or the sets of other objects) that specific attributes of the primary and temporal objects are defined over; and views, the user-oriented objects that make the database accessible to the end users.

Taking the attribute date of birth (DOB) as an example, there are many questions that must be asked:

Must every person have a DOB?
May one person have more than one DOB?
May two people have the same DOB?
What is the largest number of DsOB a single person may have?

More generally, three simpler questions may be used to determine these characteristics:

What is the minimum number of properties with this attribute that a single object may have?
What is the maximum number of properties with this attribute that a single object may have?
Must the properties with this attribute be unique for every single object in the set?

Simply stating that the minimum number of properties with a specific attribute is 1 tells us that every person must have at least one property with that attribute. On the other hand if the minimum number of properties with a specific attribute is 0, then we know that properties with that attribute are not required. Similarly, the maximum number of properties with an attribute is needed. Clearly, the maximum number of properties must be at least as large as the minimum number of properties.

If the maximum number of properties is greater then the minimum number of properties then some ordering discipline needs to be specified.

Simply stating that properties with a specific attribute are unique, means that no two people may have the same value for a property with that attribute.

Lastly, the domain of the values needs to be declared. A domain may simply be a list of values, or it may be one or more sets of objects.