A composite Key can be either Primay or Unique Key
More then One Key columns are said to be composite keys

Candidate Key(Primary Key) is a Key which Maintains the Row 
Uniue .Can be defined based on the Entity


Alternate Key or Unique Key is similar to PK ,except it 
accepts null Values .So that the records can still be 
entered submitting null values to this attribute.

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.

candidate key:-A candidate key uniquely identifies the rows
in a table.there can be more than 1 candidate key in a 
table, in which one should be a primary key.

alternate key:-In the candidate key the key which is not 
primary key is the alternate key.

composite key:- A key formed bythe combination of 2 or more 
columns is the composite key.

A candidate key is able to identify a unique tuple in a 
relation through an attribute or attribute set.

A foreign key makes sure that a tuple in a relation does 
not match a tuple in another relation which does not exist

Candidate Key.....???? Help me out..!!!!!

A candidate key is one that can identify each row
of a table uniquely. Generally a candidate key becomes the
primary key of the table. If the table has more than one
candidate key, one of them will become the primary key, and
the rest are called alternate keys.
           A key formed by combining at least two or more
columns is called composite key.