Identifying vs. Non-Identifying Relationships

An independent entity is one where the primary key contains only attributes that belong to that entity. A dependent entity is one where the primary key contains at least one attribute from another entity. In relational schemes, most notations depict independent entities on the data modeling diagram as rectangles and dependent entities as rectangles with rounded corners

In the student example shown in the Dependent and Independent Entity figure, Student and Course are independent entities and Registration is a dependent entity.

Dependent entities have at least one identifying relationship. An identifying relationship is one where the primary key of the parent (the entity on the one side of the relationship) is migrated as a foreign key to the child's primary key, as can be seen with the relationship from Student to Registration, and from Course to Registration. In non-identifying relationships, the primary key of the parent is migrated as a non-primary foreign key attribute to the child.

Relational

First articulated by Dr. Edward Codd in 1970, relational theory provides a systematic way to organize data so that they reflected their meaning (Codd, 1970). This approach had the additional effect of reducing redundancy in data storage. Codd's insight was that data could most effectively be managed in terms of two-dimensional relations. The term relation was derived from the mathematics (set theory) upon which his approach was based.

The design objectives for the relational model are to have an exact expression of business data and to have one fact in one place (the removal of redundancy). Relational modeling is ideal for the design of operational systems, which require entering information quickly and having it stored accurately.

There are several different kinds of notation to express the association between entities in relational modeling, including Information Engineering (IE), Integration Definition for Information Modeling (IDEF1X), Barker Notation, and Chen Notation. The most common form is IE syntax, with its familiar tridents or 'crows feet' to depict cardinality, see figure IE Notation.