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A correlated subquery is an inner subquery which is referenced by the main outer query such that the inner query is considered as being executed repeatedly.

Example:
----Example of Correlated Subqueries
USE AdventureWorks;
GO
SELECT e.EmployeeID
FROM HumanResources.Employee e
WHERE e.ContactID IN
(
SELECT c.ContactID
FROM Person.Contact c
WHERE MONTH(c.ModifiedDate) = MONTH(e.ModifiedDate)
)
GO

Here e.ModifiedDate-->Outer Query field refers in Inner Query.

A noncorrelated subquery is subquery that is independent of the outer query and it can executed on its own without relying on main outer query.

Example:
----Example of Noncorrelated Subqueries
USE AdventureWorks;
GO
SELECT e.EmployeeID
FROM HumanResources.Employee e
WHERE e.ContactID IN
(
SELECT c.ContactID
FROM Person.Contact c
WHERE c.Title = 'Mr.'
)
GO

Here 'Mr.'--->Outer Query field doesn't refers in Inner Query.