Legend for data models

The referential integrity relationship between database tables is illustrated using the following symbols:

A rounded, dotted rectangle represents a weak dependent entity. Instances of a weak dependent entity cannot be uniquely identified without determining its relationship to other entities. The primary key to an instance of a weak dependent entity consists entirely of foreign keys.

Example: A description table for any object is a weak dependent entity. The description cannot exist by itself and the primary key to it is the primary key of the object and the language of the description.

A rounded, solid rectangle represents a strong dependent entity. Instances of a strong dependent entity cannot be uniquely identified without determining its relationship to other entities. The primary key to an instance of a strong dependent entity contains foreign keys, but does not consist entirely of foreign keys.

Example: A vehicle registration table. The primary key consists of the car manufacturer number and the license number of the owner.

A rectangle represents an independent entity. Instances of an independent entity can be uniquely identified without determining its relationship to other entities. The primary key to an instance of an independent entity does not contain any foreign keys.

Example: An employee record. Employee ID is the primary key of this entity.

A solid relationship line represents an identifying relationship. An identifying relationship occurs when instances of the child entity are identified through association with a parent entity. The primary key attributes of the parent entity become primary key attributes of the child.

Example: A relationship between the employee table (parent) and the payroll table (foreign). Employee ID is the primary key of the employee table. However it is also both the primary key and a foreign key of the payroll table.

A dotted relationship line with a solid relationship symbol represents a mandatory non-identifying relationship. A mandatory non-identifying relationship occurs when key attributes from the parent entity must become non-key attributes of the child entity. That is, the inherited non-key attributes of the child entity in this relationship cannot be NULL.

Example: A relationship between the department table (parent) and the employee table (foreign). Each employee record has the department number as a foreign key.

A dotted relationship line with a solid and open relationship symbol represents an optional non-identifying relationship. An optional non-identifying relationship occurs when foreign keys from the parent entity can become non-key attributes of the child entity. That is, the inherited non-key attributes of the child entity in this relationship can be NULL.

Example: A relationship between the office cubicle table (parent) and the employee table (foreign). The employee can get a cubicle number as a foreign key or or the cubicle number can be null if the employee works at home or away from the office.

A relationship line that links back to the original table indicates a recursive relationship. Recursive relationships must be optional non-identifying relationships.

Example: A relationship where the parent is the employee table and the child is also the employee table. The relationship is in the form of a foreign key with the name "Manager_ID". This shows that the manager is an employee who can manage other employees.