What Is A Key? Describe Different Types Of Keys Used In Database?,define a relationship,

-A key is a single or combination of multiple fields.

-Its purpose is to access or retrieve data rows from table according to the requirement.

-The keys are defined in tables to access or sequence the stored data quickly and smoothly.

-They are also used to create links between different tables.

Primary Key

The attribute or combination of attributes that uniquely identifies a row or record in a relation is known as primary key.

Secondary key
-A field or combination of fields that is basis for retrieval is known as secondary key.

-Secondary key is a non-unique field. One secondary key value may refer to many records.

Candidate Key or Alternate key

-A relation can have only one primary key.

-It may contain many fields or combination of fields that can be used as primary key.

-One field or combination of fields is used as primary key.

-The fields or combination of fields that are not used as primary key are known as candidate key or alternate key.
-Composite key or concatenate key
A primary key that consists of two or more attributes is known as composite key.

Sort Or control key

-A field or combination of fields that is used to physically sequence the stored data called sort key. It is also known s control key.

Foreign Key

-A foreign key is an attribute or combination of attribute in a relation whose value match a primary key in another relation.

-The table in which foreign key is created is called as dependent table.

-The table to which foreign key is refers is known as parent table.