Entities

The relational database architecture is based upon the relationships of sets. Sets contain many members. Sets are discovered during the analysis process where they are referred to as entities. Entities are named as some type of an organization (Customer), person (Employee), activity (Project), location (Address) or thing (Product). Entities contain many members. For example, the Customer entity may contain thousands of members.

All entities have the same composition. They have a primary identifier, the value of which uniquely identifies each member of the set and attributes, the values of which describe the member. During data analysis, the entity is represented in the data model as a single object.

When a relationship exists between entities, the analyst places a Relation between the related entities. This object is referred to as an Associative Entity, Junction Entity, Intersection Entity or Bridge Entity. It just depends on whose book you have read. They are all the same things.

A relation (child) contains the identifiers of both related entities (parents). A relation is bi-directional and contains attributes that describe, date, qualify and quantify the relationship that exists between the entities. The structure shown above shows how the relationship between any two entities produces two occurences.

The first row describes the relationship between Entity A and Entity B. Now, by swapping the identifiers, the second row describes the relationship between Entity B and Entity A

Identifier Identifier Relationship
Mary Smith John Smith Mother
John Smith Mary Smith Son

In a data model, the connector represents the location of the primary identifiers and foreign identifiers. The entity end contains a primary identifier and is said to be "the one end of the relation." The relation end contains foreign identifiers and is said to be "the many end of the relation." There are many different symbols and techniques used to represent the many end in a diagram. All data model relationships are "one-to-many".

All entities have the same composition. They have a primary identifier, the value of which uniquely identifies each member and attributes, the values of which describe the member. During data analysis, the entity is represented in the data model as a single object (shape).

A note on which symbol, symbolism, methodology or CASE tool is a wasted effort. CASE tools are usually assigned to us and the tool describes the objects and connectors.

Pick a set within the first or second hour agree to agree and get on with the work.