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University of California, Davis

Application Development Methodology

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Development Technique Concepts

Subtype

A category of entities of an entity type that either:

* Has different attributes than the remaining entities

* Shares in different relationships than the remaining entities

* Shares different processes than the remaining entities

A collection of entities to which a specific definition and common predicates apply.

Subdivisions of the Entity Type EMPLOYEE

An entity subtype is a subdivision or further specialization of an entity type. For example, the entity type EMPLOYEE might be subdivided into:

* ADMINISTRATOR, EDITORIAL STAFFER and PRINT ROOM STAFFER

* POTENTIAL, CURRENT and FORMER EMPLOYEE

* TRAINEE, PERMANENT and FREELANCE EMPLOYEE

* MALE, FEMALE EMPLOYEE

Making a Subset into a Subtype

If these are useful subsets of the population of EMPLOYEE entities, it is worth recording them as subtypes of EMPLOYEE, defining what is special about the subtypes and analyzing their additional attributes and relationships.

For example, of all EMPLOYEEs, only EDITORIAL STAFFERs can edit and proofread BOOKs and commission AUTHORs, so there are additional relationships to the subtype EDITORIAL STAFFER. Only EDITORIAL STAFFERs have Subject Experience and relevant Qualifications. These are additional attributes of the subtype.

Note that EDITORIAL STAFFER still has all the attributes and relationships of EMPLOYEE.

Optional Subtype Classification

You may find that VICE PRESIDENT is a useful subtype of EMPLOYEE. Some EMPLOYEEs are VICE PRESIDENTs; most are not. Therefore, membership of this partitioning, which happens to have only one useful subtype, is optional. In the case of ADMINISTRATORs, EDITORIAL STAFFERs and PRINT ROOM STAFFERs, an EMPLOYEE has to be classified into one (and only one) of the subtypes. In this case, membership of the partitioning is mandatory. Describe this classification as fully enumerated.

An optional classification is the equivalent of defining an "other" subtype in a partitioning. For example, you could define a mandatory classification for EMPLOYEE to be VICE PRESIDENT or OTHER EMPLOYEE (i.e., an employee must be either a vice president or another type of employee that you have not specified).

When participation in a partitioning is optional, describe the partitioning as not fully enumerated. This implies that the population of entities existing for the entity type is not completely covered by the subtypes defined within the partitioning (i.e., some lie outside and are not counted within it).

The parent entity type must contain an attribute or combination of attributes from which one can determine which subtype applies to each entity. These attributes are called 'classifying' attributes. For instance, if EMPLOYEEs are either 'hourly' or 'salaried' then an attribute type called 'employee type' that takes on a value of 'H' or 'S' would be a classifying attribute.