Subquery is used to get information from a table based on 
unknown conditions.
If select stmt not perform entire task using 
selection,projection,and joins then we can perform that 
type of task using sub queries.

A subquery is a SELECT statement that is nested within 
another T-SQL statement. A subquery SELECT statement if 
executed independently of the T-SQL statement, in which it 
is nested, will return a result set. Meaning a subquery 
SELECT statement can standalone and is not depended on the 
statement in which it is nested.

Sometimes the criteria for determining which set of records 
will be affected by a SELECT, UPDATE, DELETE and/or INSERT 
statement cannot be obtained by hard coding the selection 
criteria. Occasionally there is a need to use the results 
of a SELECT statement to help determine which records are 
returned or are affected by a T-SQL statement. When a 
SELECT statement is used inside another statement, the 
inside SELECT statement is known as a subquery.

sub query is part of main query.this useful we retrive data
from the another table.