Answer Posted / rajkumar
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.
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