What is alternate, artificial, compound and natural key?

Alternate Key: Excluding primary keys all candidate keys are known as Alternate Keys.

Artificial Key: If no obvious key either stands alone or compound is available, then the last resort is to, simply create a key, by assigning a number to each record or occurrence. This is known as artificial key.

Compound Key: When there is no single data element that uniquely defines the occurrence within a construct, then integrating multiple elements to create a unique identifier for the construct is known as Compound Key.

Natural Key: Natural key is one of the data element that is stored within a construct, and which is utilized as the primary key.