selecting rows with one same and one different column

Question

I am trying to list the Company Name of all U.S.-based customers who are NOT located in the same state (or region) as any of the employees. It should return 10 rows. I have used the following query for this

SELECT Customers.CompanyName
FROM Customers
WHERE Customers.Country='USA'
  AND Customers.Region IN
    (SELECT Customers.Region
     FROM Customers
     GROUP BY Customers.Region HAVING COUNT(Customers.Region)=1);

but it returns only 6.

The database is northwind. **Company name

---

here is the output
Old World Delicatessen
Let's Stop N Shop
Save-a-lot Markets
The Cracker Box
Rattlesnake Canyon Grocery
Split Rail Beer & Ale**

Answer 1

Try this

SELECT CompanyName
  FROM Customers
 WHERE Country='USA' AND 
       Region NOT IN
           (SELECT DISTINCT Region
              FROM Employees 
             WHERE Region IS NOT NULL);

Output

CompanyName
----------------------------------------
Great Lakes Food Market
Hungry Coyote Import Store
Let's Stop N Shop
Lonesome Pine Restaurant
Old World Delicatessen
Rattlesnake Canyon Grocery
Save-a-lot Markets
Split Rail Beer & Ale
The Big Cheese
The Cracker Box

(10 row(s) affected)

In your your question you said that you need Company Name of all U.S.-based customers who are NOT located in the same state (or region) as any of the employees. But in your query you were trying to get regions from Customers table instead.

UPDATE: I believe that you know that but just in case:

The explicit check for NULL values in the subquery (WHERE Region IS NOT NULL) is crucial. Without it the query will run successfully but won't return any rows.

SELECT CompanyName
  FROM Customers
 WHERE Country='USA' AND 
       Region NOT IN
           (SELECT DISTINCT Region
              FROM Employees);

Output:

CompanyName
----------------------------------------

(0 row(s) affected)

I guess that exercise that you were trying to accomplish was meant to make sure that you know about NULLs in subqueries.

Answer 2

Your query does not check Employees table at all! Check out that:

SELECT Customers.CompanyName, customers.Region
FROM Customers
WHERE Customers.Country='USA'
AND NOT EXISTS(SELECT 1 FROM Employees WHERE Employees.Region = Customers.Region)

or

SELECT Customers.CompanyName, customers.Region
FROM Customers
WHERE Customers.Country='USA'
AND  Customers.Region NOT IN (SELECT DISTINCT Employees.Region FROM Employees WHERE Employees.Region IS NOT NULL)

Both queries return:

Great Lakes Food Market         OR
Hungry Coyote Import Store      OR
Let's Stop N Shop               CA
Lonesome Pine Restaurant        OR
Old World Delicatessen          AK
Rattlesnake Canyon Grocery      NM
Save-a-lot Markets              ID
Split Rail Beer & Ale           WY
The Big Cheese                  OR
The Cracker Box                 MT