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. 

candidate key is a single key of each tuple.
alternative key is secondary key . for example(A entity 
STUDENT an attribute of NAME,CLASS,ROLL.ID)the roll no is 
primary key but id is alternate key.

A key formed combined of to or more columns called comosite 
key.

Any attribute that is uniquely identify a row in a table is 
candidate key for the table. We select one of the candidate 
key as Primary key. All candidate keys which are not chosen 
as "primary key" are Alternate keys. The key which uniquely 
identify the rows of the table  and which is made up of 
more than one attribute is called Composite key.
As for Example:In a class we have to select Class Monitor. 
So A, B,C and D stand for that post. So A,B,C and D are 
candidate for monitor so these are candidate key. We select 
B as monitor so B is primary key and A,C and D can be 
monitor but not selected as a monitor so they are 
alternative choise. So A,C and D are alternate key.
When two students of class work together in a project than 
they are composite key for the class.

I TRIED TO EXPLAIN THESE KEY, IF YOU LIKE PLEASE DO 
RESPONSE. iF YOU LIKE TO ASK ANY OTHRE QUE RELATED TO SQL 
SERVER YOU CAN..

Candidate Key is a Key which identifies each row of a table 
uniquely. Generally a Candidate Key becomes a Primary Key 
of the table. If table has more than one Candidate Keys  
then one is called Primary key and rest are called 
Alternative Keys. 
A key is formed by two or more columns is called Composite 
Keys.

Candidate key: If any unique data column(s)in table is 
called candidate key. may be one or more unique data column
(s).We can select any one unique (candidate key column) as 
a primary key.

Alternate key: If one table contains more then one 
candidate keys, remining candidate keys column(s) which is 
not selected as a primary key is called Alternate Key.

Composite key: One primary key Contains more then one   
columns is called Composite key.

A CANDIDATE KEY ISA PRIMERY KEY. OTHER ARE ALTERNATE KEY

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.

How do we specify an ALTERNATE KEY while creating a table?

A candidate key is a combination of attributes that can be uniquely used to identify a database record without any extraneous data. Each table may have one or more candidate keys. One of these candidate keys is selected as the table primary key.