Need subquery for conditional output - sql-server

Question

I am working with a set of vehicle data that uses the following query:
SELECT
VIN_NUM AS [Registration VIN]
,REGION_IND AS [Location of Registration]
,REG_CHANGE AS [Changed Location Since Last Check]
,CASE
WHEN REG_CHANGE = '' THEN REGION_IND
ELSE REG_CHANGE
END AS [Final Location]
FROM
dbo.All_Tests
WHERE
VIN_NUM LIKE '1FM%' AND
CASE
WHEN REGION_IND = '1' THEN 'Upstate'
WHEN REGION_IND = '2' THEN 'Downstate'
ELSE 'Unknown'
END = 'Downstate'
The query pulls from a table a vehicle VIN (VIN_NUM) and whether it is located in one of two regions (REGION_IND), "1" or "2". It also pulls a column, "REG_CHANGE" checking if the vehicle registration has changed location between the two regions since last report. All three come from the same table.
REG_CHANGE is blank (not NULL) if there was no change, and contains the new region location, '1' or '2', if there was a change. This is used in a CASE statement with REGION_IND to give a current location to all vehicles in the database, alias name [Final Location].
The code works if I want the original regions since REGION_IND is a table column. However, I can't use [Final Location] because WHERE statements don't allow aliases. I'm thinking this would be a subquery construct within the SELECT columns, but I'm not certain how it would be structured.
Does anyone have any suggestions?

Answer 1

A useful approach for this is to use an applyoperator within the fromclause which then does permit the use of that column alias within the where clause:
SELECT
VIN_NUM AS [registration vin]
, REGION_IND AS [location of registration]
, REG_CHANGE AS [changed location since last check]
, ca.[final location]
FROM dbo.All_Tests
CROSS APPLY (
SELECT
CASE
WHEN REG_CHANGE = '' THEN REGION_IND
ELSE REG_CHANGE
END AS [final location]
) ca
WHERE VIN_NUM LIKE '1FM%'
AND ca.[final location] = 'Downstate'
Any further uses of àpply that follow can also use these column aliases.
btw: Although a SQL select query starts with the select clause, that clause performed after the from and where clauses. So defining a column alias in the from clause makes that alias available much earlier in the execution of the query.

Answer 2

you can write your where like below
SELECT
VIN_NUM AS [Registration VIN]
,REGION_IND AS [Location of Registration]
,REG_CHANGE AS [Changed Location Since Last Check]
,CASE
WHEN REG_CHANGE = '' THEN REGION_IND
ELSE REG_CHANGE
END AS [Final Location]
FROM
dbo.All_Tests
WHERE
VIN_NUM LIKE '1FM%' AND
(
(REGION_IND = '1' and REG_CHANGE ='Upstate') OR
(REGION_IND = '2' and REG_CHANGE ='Downstate') OR
(REG_CHANGE = 'Downstate')
)