Candidate key and Alternate key ,Composite 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. 
Composite Key: A key formed by combining at least two or more columns is called composite key.

Generally Candidate key is irreducible set of Super key, means you can't reduce more to make a primary key.and you can say it set of primary key because any one of them it can be a primary key. 
And rest of all called Alternate Key.

If you are unable to understand then we take an example and try to understand you.
if any table schema  is:  Student(a,b,c,d)
then we make set of primary key like this, then Candidate Key((a,b),(c,a),(a,b,d)) here you may be confuse but no problem ,it is only set of primary key, so any set of candidate key can be make a primary key. 
And if we make a set primary key(a,b) then rest of set is called alternate key.
and Composite key only those key which contains two or more than column of the table.

Here is example of Composite Primary Key......

CREATE TABLE Persons(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
)

This example is taken from w3schools.com....

AND / OR
SELECT column_name(s)
FROM table_name
WHERE condition
AND|OR condition
ALTER TABLE
ALTER TABLE table_name 
ADD column_name datatypeor

ALTER TABLE table_name 
DROP COLUMN column_name
AS (alias)
SELECT column_name AS column_alias
FROM table_nameor

SELECT column_name
FROM table_name  AS table_alias
BETWEEN
SELECT column_name(s)
FROM table_name
WHERE column_name
BETWEEN value1 AND value2
CREATE DATABASE
CREATE DATABASE database_name
CREATE TABLE
CREATE TABLE table_name
(
column_name1 data_type,
column_name2 data_type,
column_name2 data_type,
...
)
CREATE INDEX
CREATE INDEX index_name
ON table_name (column_name)or

CREATE UNIQUE INDEX index_name
ON table_name (column_name)
CREATE VIEW
CREATE VIEW view_name AS
SELECT column_name(s)
FROM table_name
WHERE condition
DELETE
DELETE FROM table_name
WHERE some_column=some_valueor

DELETE FROM table_name 
(Note: Deletes the entire table!!)

DELETE * FROM table_name 
(Note: Deletes the entire table!!)
DROP DATABASE
DROP DATABASE database_name
DROP INDEX
DROP INDEX table_name.index_name (SQL Server)
DROP INDEX index_name ON table_name (MS Access)
DROP INDEX index_name (DB2/Oracle)
ALTER TABLE table_name
DROP INDEX index_name (MySQL)
DROP TABLE
DROP TABLE table_name
GROUP BY
SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING
SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value
IN
SELECT column_name(s)
FROM table_name
WHERE column_name
IN (value1,value2,..)
INSERT INTO
INSERT INTO table_name
VALUES (value1, value2, value3,....)or

INSERT INTO table_name
(column1, column2, column3,...)
VALUES (value1, value2, value3,....)
INNER JOIN
SELECT column_name(s)
FROM table_name1
INNER JOIN table_name2 
ON table_name1.column_name=table_name2.column_name
LEFT JOIN
SELECT column_name(s)
FROM table_name1
LEFT JOIN table_name2 
ON table_name1.column_name=table_name2.column_name
RIGHT JOIN
SELECT column_name(s)
FROM table_name1
RIGHT JOIN table_name2 
ON table_name1.column_name=table_name2.column_name
FULL JOIN
SELECT column_name(s)
FROM table_name1
FULL JOIN table_name2 
ON table_name1.column_name=table_name2.column_name
LIKE
SELECT column_name(s)
FROM table_name
WHERE column_name LIKE pattern
ORDER BY
SELECT column_name(s)
FROM table_name
ORDER BY column_name [ASC|DESC]
SELECT
SELECT column_name(s)
FROM table_name
SELECT *
SELECT *
FROM table_name
SELECT DISTINCT
SELECT DISTINCT column_name(s)
FROM table_name
SELECT INTO
SELECT *
INTO new_table_name [IN externaldatabase]
FROM old_table_nameor

SELECT column_name(s)
INTO new_table_name [IN externaldatabase]
FROM old_table_name
SELECT TOP
SELECT TOP number|percent column_name(s)
FROM table_name
TRUNCATE TABLE
TRUNCATE TABLE table_name
UNION
SELECT column_name(s) FROM table_name1
UNION
SELECT column_name(s) FROM table_name2
UNION ALL
SELECT column_name(s) FROM table_name1
UNION ALL
SELECT column_name(s) FROM table_name2
UPDATE
UPDATE table_name
SET column1=value, column2=value,...
WHERE some_column=some_value
WHERE
SELECT column_name(s)
FROM table_name
WHERE column_name operator value