Posted by tac_admin, November 22, 2011

Data Entities and Attributes

An Entity represents a real world object or a thing that can stand on its own independently and can be identified uniquely. Attributes are the properties of these entities. Data attributes are assigned to entity types.

Example of data for an employee database:

Employee ID – Attribute

Name – Attribute

SSN – Attribute

Address – Entity (group of data)

Salary – Attribute

Job Title – Attribute

Tax Info – Entity

Benefits – Entity

Reviews – Entity

Vacation – Attribute

Hire Date – Attribute

Age – Attribute

Gender – Attribute

The primary key of a relational table uniquely identifies each record in the table. It can either be a normal attribute that is guaranteed to be unique (such as Social Security Number in a table with no more than one record per person) or it can be generated by the DBMS (such as a globally unique identifier, or GUID, in Microsoft SQL Server). Primary keys may consist of a single attribute or multiple attributes in combination.
A primary key is always unique and mandatory. If it’s not unique and mandatory, it can’t be a primary key in the database.

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