Invalid Column Name in Select Statement

Question

Following query throws

Invalid column name WIP_Aging"

Query:

Select 
    ID, Assembly, Last_Accessed, DATEDIFF(day,Last_Accessed,GETDATE()) as WP_Aging 
from 
    Details 
where 
    WP_Aging >= 2

Answer 1

Unless you create a CTE or Sub-Select you can't refer to the aliased column name ( WP_Aging ) directly. Instead you have to repeat the expression:

Select 
    ID, 
    Assembly,  
    Last_Accessed, 
    DATEDIFF(day, Last_Accessed, GETDATE()) as WP_Aging 
from 
    Details 
where 
    DATEDIFF(day, Last_Accessed, GETDATE()) >= 2

Here's the CTE version:

;with cteAging as
(
    Select 
        ID, 
        Assembly,  
        Last_Accessed, 
        WP_Aging = DATEDIFF(day, Last_Accessed, GETDATE())
    from 
        Details 
)
select
    ID, 
    Assembly,  
    Last_Accessed, 
    WP_Aging 
from
    cteAging 
where 
    WP_Aging >= 2

Answer 2

It's the way the engine works. The WHERE clause is done before the SELECT. You basically "created" a column named WP_aging during the SELECT. If you think about it it makes sense. You want to reduce the rows before you go and get the data want to see. So it does the WHERE and reduces the set first.