Can I Use Column Aliases in WHERE Clauses?

Refer column alias in WHERE clause

Referencing column aliases in the WHERE clause can sometimes cause errors when processing database queries. This is because aliases are usually applied after the WHERE clause is evaluated. Consider the following query:

SELECT logcount, logUserID, maxlogtm
   , DATEDIFF(day, maxlogtm, GETDATE()) AS daysdiff
FROM statslogsummary
WHERE daysdiff > 120

In this query, the error occurs because "daysdiff" is a column alias created in the SELECT statement. To reference an alias in a WHERE clause, you can force SQL to evaluate the SELECT statement before the WHERE clause. This can be achieved by using parentheses or common table expressions (CTE).

Bracket/Subquery:

SELECT
   *
FROM
(
   SELECT
      logcount, logUserID, maxlogtm,
      DATEDIFF(day, maxlogtm, GETDATE()) AS daysdiff
   FROM statslogsummary   
) as innerTable
WHERE daysdiff > 120

CTE:

WITH CTE AS (
   SELECT
      logcount, logUserID, maxlogtm,
      DATEDIFF(day, maxlogtm, GETDATE()) AS daysdiff
   FROM statslogsummary
)
SELECT
   *
FROM CTE
WHERE daysdiff > 120

By using parentheses or a CTE, the SELECT statement is evaluated before the WHERE clause, allowing you to reference column aliases in the WHERE clause without errors.

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