Question

for eg…

SELECT * 
  FROM ( SELECT RANK() OVER (ORDER BY stud_mark DESC) AS ranking,
                stud_id, 
                stud_name, 
                stud_mark 
           FROM tbl_student ) AS foo 
 WHERE ranking = 10

Here foo is present…actually what it does ?..

Answer 1

It is just an alias.

Aliases help you reduce the amount of text you may have to type out when writing queries.

For instance this:

SELECT customer.Name, customer.OpenDate FROM customer

Can be shortened to:

SELECT c.Name, c.OpenDate FROM customer c

In your example, it is a derived table (not a physical one) which now you can actually say:

SELECT foo.someField rather then SELECT *