Alternative manner against subquery to read the first date from a table

Question

Hi,
in order to improve the query performance, f.e. is it possible to re-write this query with a subquery:
SELECT OrderId, Customerid,
(SELECT TOP 1 Address
FROM Customers
WHERE CustomerId = Orders.CustomerId and Effective_Begin_Date <= Orders.OrderDate
ORDER BY Effective_Begin_Date desc) as CustomerAddress
FROM Orders
by using a Join?
Many thanks
P.S.: this is an example and not a real working case.

Answer 1

To be honest, I think you'd better be served with a view that calculated the begin and end date for a customers address, and then used that to join to in your queries.
DECLARE @orders TABLE (orderID INT IDENTITY, customerID INT, orderDate DATE)
DECLARE @customers TABLE (customerID INT, address INT, effective_Begin_Date DATE)
INSERT INTO @customers (customerID, address, effective_Begin_Date) VALUES (1, 1, '2015-01-01'),(2, 2, '2015-02-01'),(3, 3, '2015-02-02'), (1, 4, '2015-02-01')
INSERT INTO @orders (customerID, orderDate) VALUES (1, '2015-01-01'),(2, '2015-02-01'),(2, '2015-02-01'),(2, '2015-02-02'),(3, '2015-01-01'),(3, '2015-02-02'),(1, '2015-02-05')
;WITH customerAddress AS (
SELECT c1.customerID, c1.effective_Begin_Date, DATEADD(DAY,-1,c2.effective_Begin_Date) AS effective_End_Date, c1.address
FROM @customers c1
LEFT OUTER JOIN @customers c2
ON c1.customerID = c2.customerID
AND c2.effective_Begin_Date = (SELECT MIN(effective_Begin_Date) FROM @customers WHERE customerID = c1.customerID and effective_Begin_Date > c1.effective_Begin_Date)
SELECT orderID, c1.customerID, ca.address, ' ', *
FROM @customers c1
INNER JOIN customerAddress ca
ON c1.customerID = ca.customerID
AND c1.address = ca.address
LEFT OUTER JOIN @orders o
ON c1.customerID = o.customerID
AND o.orderDate BETWEEN ca.effective_Begin_Date AND COALESCE(ca.effective_End_Date,CURRENT_TIMESTAMP)
ORDER BY c1.customerID, o.orderID
SELECT OrderId, Customerid,
(SELECT TOP 1 Address
FROM @Customers
WHERE CustomerId = o.CustomerId
AND Effective_Begin_Date <= o.OrderDate
ORDER BY Effective_Begin_Date desc) AS CustomerAddress
FROM @Orders o
I've mocked up your customer and order tables here, and used a CTE in place of a view. There's a couple of reasons to go this route, but the primary one is once the view is created you have one object thats controling the logic - one place to troubleshoot,
one place to change.