How Correlated Subquery work with JOIN

Question

Hi, There,
I use Northwind database, and the codes below executed good.
      USE Northwind
      SELECT c.Country, o.OrderID, o.CustomerID, o.OrderDate
      FROM Orders o
      INNER JOIN Customers c
      ON o.CustomerID=c.CustomerID
      WHERE o.CustomerID IN(SELECT c.CustomerID FROM Orders WHERE Country ='Germany')
However, in the subquery,
(SELECT c.CustomerID FROM Orders WHERE Country ='Germany')
, if I replaced the "Orders" by "Customers",
, then the Country will not be filtered to Germany as I need.
I am curious to know why it cannot filter the Country in that way.
Appreciate you for your explanation!!

Answer 1

Hi TryToFly,
Based on my test, I can reproduce the same issue in my local environment.
To compare the difference between those two queries, we can see the Execution plans corresponding to the queries. Based on the Execution plans, I find this:
The first query: SELECT c.Country, c.CustomerID  FROM Customers c   WHERE Country =N'Germany'
                       SELECT o.OrderID, o.CustomerID, o.OrderDate, o.CustomerID  FROM Orders o
                       SELECT c.Country, o.OrderID, o.CustomerID, o.OrderDate  FROM Orders o  INNER JOIN Customers c  ON o.CustomerID=c.CustomerID
The second query: SELECT c.Country, c.CustomerID  FROM Customers c   
                            SELECT c.Country, c.CustomerID  FROM Customers c   
                            SELECT c.Country, o.OrderID, o.CustomerID, o.OrderDate  FROM Orders o  INNER JOIN
Customers c  ON o.CustomerID=c.CustomerID
                            SELECT c.CustomerID  FROM Customers c   WHERE Country =N'Germany'
                            SELECT c.Country, o.OrderID, o.CustomerID, o.OrderDate  FROM Orders o INNER JOIN Customers
c  ON o.CustomerID=c.CustomerID  WHERE o.CustomerID IN(SELECT c.CustomerID FROM Customers WHERE Country ='Germany')
According to the Execution plans, the results make sense.
Thanks,
Katherine Xiong
Katherine Xiong
TechNet Community Support