As database developer, you must have ever written 
subqueries in either SQL Server or any other database 
platform. This article will describe shortly about types of 
subquery.

  
A subquery is inner query that will be used by outer query. 
A subquery could return scalar value or a series of value. 
Based on query dependency, a subquery could be self-
contained or correlated one. A self-contained subquery is a 
subquery independent from outer query, and correlated 
subquery is a subquery that is referencing to a row in 
outer query.

  
Self-contained subquery is easier to debug than correlated 
subquery. And it is executed only once whereas correlated 
subquery is executed once for each row of outer query.

  
Sample of self-contained subquery that returns scalar value 
is shown below :

  
Select customerid

From dbo.orders

Where employeeid = (select employeeid from dbo.employees 
where lastname = ‘RAJ’);

  
This query returns customers list whose employee is RAJ.