How can I use a calculated field in SQL Server query WHERE condition? (Invalid column name error)

Question

I am not so into Microsoft SQL Server and I have the following problem.

I have this query:

SELECT 
     [ID]
    ,[Numero protocollo]
    ,[Anno]
    ,[IdUor]
    ,[Protocollista]
    ,[Protocollato]
    ,[avorato]
    ,[Errore]
    ,[CopiaConoscenza]
    ,[Inoltro]
    ,[DataProtocollo]
    ,GETDATE () AS [Today]
    ,ABS(DATEDIFF(DAY, GETDATE (), [DataProtocollo])) AS [NumberOfDays]
FROM 
    [PROT_INOLTRO]
WHERE 
    [PROT_INOLTRO].IdUor = 1
    AND [PROT_INOLTRO].Protocollista = 'i:0#.w|iwgroupnet\anobili'
    AND [NumberOfDays] < 15
ORDER BY 
    ID DESC

As you can see, I added 2 calculated columns: [Today] and [NumberOfDays].

As you can see I am filtering using 3 WHERE conditions.

My problem is that I need to filter using the [NumberOfDays] column that is a calculated column, and not a column defined directly in the PROT_INOLTRO table.

Executing this query I get the following error message:

Msg 207, Level 16, State 1, Line 23
Invalid column name 'NumberOfDays'

Why can't I use this calculated column in my WHERE condition? How can fix my query to obtain this behavior?

Answer 1

Use like this:

...
AND ABS(DATEDIFF(DAY, GETDATE (), [DataProtocollo]))<15