Access alias in CASE statement - sql

Question

I am trying to create a column called DateStartedStatus that utilizes a previously aliased column to compute its value. It should use CurrentStatus to output a value and an error is showing that says "Invalid column name 'CurrentStatus'". How can I access that alias in the below case statement?
SELECT p.[ID]
,p.[Name] as 'ProcurementName'
,p.[FundingDocumentNumber] as 'FundingDocumentNumber'
,p.[Status]
,p.[Comments] as 'Comments'
,p.[isSAVE]
,p.[InWorkDate]
,p.[RoutedDate]
,p.[FundsCertifiedDate]
,p.[AwardedDate]
,p.[TransactionType]
,p.[FNMSStatus]
,p.[Closed]
,p.[Archived]
,p.[Cancelled]
,(CASE
WHEN p.[Status] = 'In Work' THEN 'Pending'
ELSE p.[Status]
END) as CurrentStatus
,(CASE
WHEN CurrentStatus = 'Awarded' THEN p.AwardedDate <-- fails here CurrentStatus not a column
END) as DateStartedStatus
,(SELECT SUM(TotalCost)
FROM ProcurementsRequestLineItems subprlis
LEFT JOIN RequestLineItems subrli ON subprlis.RequestLineItemID = subrli.ID
WHERE ProcurementID = p.ID) as TotalCost
FROM Procurements p
WHERE p.Closed = 0 AND p.Archived = 0;

Answer 1

Use a subquery as suggested by leftjoin, or move the CurrentStatus logic to a CTE. I prefer CTE as they are more legible to me, but I know many prefer a subquery as it is right in the middle of the code, and in a longer query or one with many CTE's that can be a more legible route.
WITH CurrentStatus
AS
(
SELECT
... -- at least one JOIN'able column back to the main query
,(CASE
WHEN p.[Status] = 'In Work' THEN 'Pending'
ELSE p.[Status]
END) as CurrentStatus
FROM ...
)

Answer 2

Using subqueries like this
select ... CASE WHEN CurrentStatus ....
from
( --calculate Current_status here
select ....
CASE
WHEN p.[Status] = 'In Work' THEN 'Pending'
ELSE p.[Status]
END) as CurrentStatus
...
) s
Do not worry, subquery will not add computational complexity, optimizer will remove it if possible.
Another way is nested CASE expressions (query is not readable):
case when case ... some logic here ... end = 'Awarded'
then ...
end

Answer 3

For SQL Server, I would use a CROSS APPLY instead of an subquery, because I prefer it for readability. For one-condition evaluation, I use IIF instead of CASE.
SELECT p.[ID], p.[Name] AS [ProcurementName], p.[FundingDocumentNumber] AS [FundingDocumentNumber],
p.[Status], p.[Comments] AS [Comments], p.[isSAVE], p.[InWorkDate], p.[RoutedDate], p.[FundsCertifiedDate],
p.[AwardedDate], p.[TransactionType], p.[FNMSStatus], p.Closed, p.[Archived], p.[Cancelled],
cur.CurrentStatus, start.DateStartedStatus, tot.TotalCost
FROM Procurements AS p
CROSS APPLY (SELECT IIF(p.[Status] = 'In Work', 'Pending', p.[Status]) AS CurrentStatus) AS cur
CROSS APPLY (SELECT IIF(cur.CurrentStatus = 'Awarded', p.AwardedDate, null) AS DateStartedStatus) AS start
CROSS APPLY (
SELECT SUM(TotalCost) AS name
FROM ProcurementsRequestLineItems AS subprlis
LEFT JOIN RequestLineItems AS subrli ON subprlis.RequestLineItemID = subrli.ID
WHERE ProcurementID = p.ID
) AS tot
WHERE p.Closed = 0 AND p.Archived = 0;
I would also avoid using the reserved word "Status" as a column identifier.