SQL Server: Multi-valued subquery with nesting

Hi Friends,

In a multi-valued, self-conatined subquery, the inner query (also called as the inner select) returns multiple values. The outer query consumes and validates the list of values returned by the subquery using IN clause. Subqueries can also be nested. Here is an example:-

Use AdventureWorks;
SELECT LastName, FirstName
FROM Person.Contact
    (SELECT ContactID
     FROM HumanResources.Employee
     WHERE EmployeeID IN
        (SELECT SalesPersonID
         FROM Sales.SalesPerson))

The innermost query returns the sales person IDs. The query at the next higher level is evaluated with these sales person IDs and returns the contact ID numbers of the employees. Finally, the outer query uses the contact IDs to find the names of the employees.


The above query can also be written as a join:

Use AdventureWorks;
SELECT LastName, FirstName
FROM Person.Contact c
INNER JOIN HumanResources.Employee e
ON c.ContactID = e.ContactID
JOIN Sales.SalesPerson s
ON e.EmployeeID = s.SalesPersonID

So friends, what about performance? Run both the queries as a batch and see the relative costs. Which one you think performs better? Comments?




About Amit Bansal

Amit Bansal is always brainstorming around SQL Server. Despite working with SQL since 1997, he is amazed that he keeps learning new things every single day. SQL Server is AB's first love, and his wife does not mind that. He tries to share as much and spreads the SQL goodness. Internals and Performance Tuning excites him, and also gives him sleepless nights at times, simply because he is not a genius, but quite a hard worker and does not give up. It has been a long and exciting journey since 1997, you can read here: http://sqlmaestros.com/amit-bansal/ He is on Twitter: https://www.twitter.com/A_Bansal

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