Referencing a calculated column in the where clause SQL

问题

This line of code is a snippet from my select statement.

frdFreedays - DateDiff(dd,conReceiptToStock,GetDate()) As FreeDaysRemaining

Below is a snippet from my where clause

and frdFreedays - DateDiff(dd,conReceiptToStock,GetDate()) <= @intFreeDays

The question I have is how can I reference the FreeDaysRemaining column and so I can compare it to @intFreeDays

I am looking for something like this

Freedays <= @intFreeDays

回答1:

In addition to Aaron's answer, you could use a common table expression:

;with cte_FreeDaysRemaining as
    (
        select
            frdFreedays - DateDiff(dd,conReceiptToStock,GetDate()) As FreeDaysRemaining
            --, more columns
        from yourtable
    )
    select
        FreeDaysRemaining
        --, more columns
    from cte_FreeDaysRemaining
    where FreeDaysRemaining <= @intFreeDays

回答2:

You can't reference an alias anywhere except ORDER BY. One workaround (aside from the obvious possibility of repeating the expression) is to put it in a derived table:

SELECT FreeDaysRemaining --, other columns
FROM
(
  SELECT frdFreedays - DATEDIFF(DAY, conReceiptToStock, GETDATE()) AS FreeDaysRemaining
    --, other columns
  FROM ...
) AS x
WHERE FreeDaysRemaining <= @intFreeDays;

来源：https://stackoverflow.com/questions/9720790/referencing-a-calculated-column-in-the-where-clause-sql