How can i move this Select Statement with Top - sql

Question

I'm trying to optimize this query to use a join instead of this sub query, but with the Top 1, I'm a little confused.
SELECT
s.ItemNumber
s.ImportKey
,(
SELECT top 1 MerchandiseGroupID
FROM MerchandiseGroup mg
WHERE s.StoreDepartment = mg.Name AND c.ClientNumber = s.ClientNumber
) as MerchandiseGroupID
FROM dbo.Source s
INNER JOIN dbo.Client c on s.ClientNumber = c.ClientNumber
INNER JOIN dbo.ClientVendor cv on s.Vendor = cv.ClientVendorName
INNER JOIN dbo.TypeClientWarehouse tw on c.WarehouseCode = tw.WarehouseCode
WHERE s.ImportDate > '2014-05-15 01:00:00.000'
What I have so far:
INNER JOIN
(
SELECT
ROW_NUMBER() OVER (PARTITION BY Name ORDER BY MerchandiseGroupID asc) rnum,
MerchandiseGroupID,
Name
FROM MerchandiseGroup mg
) mhg
ON s.StoreDepartment = mg.Name AND c.ClientNumber = s.ClientNumber
WHERE s.ImportDate > '2014-05-15 01:00:00.000' AND mg.rnum = 1

Answer 1

I think using TOP 1 is no less optimal that turning it into a join and using ROW_NUMBER(), however if the reason you want a join is so you can get other fields from the tables then use an APPLY (Your first query has no order by with the top so I have assumed it to be the same as in your ROW_NUMBER function in the JOIN attempt):
SELECT
s.ItemNumber,
s.ImportKey,
mg.MerchandiseGroupID,
mg.Name
FROM dbo.Source s
INNER JOIN dbo.Client c on s.ClientNumber = c.ClientNumber
INNER JOIN dbo.ClientVendor cv on s.Vendor = cv.ClientVendorName
INNER JOIN dbo.TypeClientWarehouse tw on c.WarehouseCode = tw.WarehouseCode
OUTER APPLY
(
SELECT top 1 MerchandiseGroupID, Name
FROM MerchandiseGroup mg
WHERE s.StoreDepartment = mg.Name
AND c.ClientNumber = s.ClientNumber
ORDER BY MerchandiseGroupID
) mg
WHERE s.ImportDate > '2014-05-15 01:00:00.000'
This approach tends to be faster than ROW_NUMBER if you only want the top 1, it is less flexible if you want specific records though, such as the 3rd record.
Not related, but becareful when using the date format you have, even though this is the ISO standard it can still throw an error:
SET DATEFORMAT DMY;
SELECT CAST('2014-05-15 01:00:00.000' AS DATETIME);
Will give you:
Msg 242, Level 16, State 3, Line 2
The conversion of a varchar data type to a datetime data type resulted in an out-of-range value.