Database Component

Entity

Something that can be identified in the users’ work environment, something the users want to track (person,place,thing or event)

Entity set or Entity Class
The collection of all Entities/Entity Instances

Attributes
Describes the Entity’s characteristics

Weak Entity
An entity that cannot be uniquely identified by its attributes alone.
-must use a foreign key in conjunction with its attributes to create a primary key.
-the foreign key is typically a primary key of an entity it is related to.
-cannot exists without another entity.
-for example, before employee dependents can be recorded, the employee must exist first

Strong Entity
An entity type is strong if its existence does not depend on some other entity.
-has a Primary key which can be used to uniquely identify its attributes
-entity can stand alone.

Primary Key
A unique key to identify a row or a tuple.
-must never contain redundant attributes 

-can never be a null value

Composite Primary Key
Composition or combination of Primary Key

Candidate Key
When any attribute or combination of attributes that could serve as a primary key is not selected to be the primary key, than that attribute or combinations of attribute that will function as a primary.

Secondary Key
Finding a specific row without using the primary key by only using other attributes.
-handy when using large database 

-non primary search key is secondary key

Foreign Key
Relationship between tables is established through the use of a foreign key.
-the primary key of one table, put into the table of another entity.