Database Keys

primary key:-  The attribute or combination of attributes
that uniquely identifies a row or record.

Foreign Key:- an attribute or combination of attribute in a
table whose value match a primary key in another table.

Composite key:- A primary key that consistsof two or more
attributes is known as composite key

candidate key:-  is a column in a table which has the
ability to become a primary key.

Alternate Key:- Any of the candidate keys that is not part
of the primary key is called an alternate key.

Alternate Key or Unique Key is similar to PK ,except it 
accepts null Values