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Entity

An entity is an object that: * has attributes that confirm its existence * is differentiable from every other entity in some way

Entity Set

An entity set is a set of entitites that share the same attributes * For example, a table containing all employees and their attributes is an entity set.

Attribute

An attribute is a value that tells us something noteworthy about the entity that it belongs to

Primary Key

A primary key is a special attribute, that helps us uniquely identify each member of the identity set. Think of it as an ID card

Example

In the Battling Civilisations videogame, one enters a stage (one of many, with their own distinct designs) with their army. Both the player's and foe's armies contain soldiers, who carry weapons. While traversing the stage, one has to watch out for traps set by the enemy's civilisation

The entities identifiable in this extract are: * Stage * Civilisation * Army * Soldier * Weapon (another Weak Entity) * Trap

The identification of an entity in E-R Database Design is very similar to the identification of a class in Object Oriented Design

Representation of an Entity

soldiersoldier_IDnameequipped_weapon
  • A rectangle represents the collective attributes of an entity set
  • The title of the entity is present in the header
  • Attributes are present in the body
  • The Primary Key is underlined