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University of California, Davis

Application Development Methodology

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Development Technique Concepts

Identifier

A predicate or set of predicates whose values distinguish each entity of a specific type from others of the same type.

Primary Identifier

The preferred means of distinguishing each entity of a type from others of the same type.

When alternative identifiers are found, designate the identifier likely to be the most useful as the primary identifier.

Ways of Distinguishing Entities

There must be at least one way of distinguishing entities of a type, or you do not have a valid entity type. It is, however, possible to have more than one way of identifying entities. Distinguish between entities of the same entity type by:

* An attribute whose value distinguishes the entities

* The existence of a relationship to another entity

* A combination of attributes and relationships

Single Attribute Value Identifier

Attribute values may be used to identify entities uniquely. A single value may suffice (e.g., "JM & Co. 1" identifies a vehicle). Alternative attribute values may also be employed. For example, if JM & Co. 1 is owned by James Martin & Co., it may also be identifiable by a unique asset number. An asset number as the identifying attribute of the entity type VEHICLE, however, is useful only for company-owned vehicles. Note also that asset number is a designed attribute.

Composite Identifier

Two or more predicated (attributes or relationships) whose values, taken in combination, uniquely identify the entities of one type.

When the values of more than one attribute are needed in combination, the identifier is composite. For example, SUPPLIER might be identified by Name and Location, as in James Martin & Co., Reston. VA.

It is possible to identify entities with relationship memberships or with a combination of relationship memberships and attributes. Relationship memberships are used when the identifying value will be found in a related entity type. In short, an entity can be identified by any combination of predicates.

For example, an ORDER ITEM may be identified by its relationship with a particular order and Item Number (an attribute of ORDER ITEM). Item Numbers may be duplicated across a set of ORDER ITEMS for different ORDERs, so the occurrences must be distinguished by finding the ORDER that they belong to. Order Number is associated with an ORDER ITEM by virtue of the relationship between the two entity types. It is in part the existence of a relationship membership that identifies the ORDER ITEM.

An entity may be identified purely by relationships. For example, the entity type PRODUCT STOCK is identified uniquely by its relationship with the PRODUCT and WAREHOUSE LOCATION entity types.