Can I Use Column Aliases in a WHERE Clause?

Refer column alias in WHERE clause

The following SQL query uses column aliases in the WHERE clause:

<code class="language-sql">SELECT
   logcount, logUserID, maxlogtm
   , DATEDIFF(day, maxlogtm, GETDATE()) AS daysdiff
FROM statslogsummary
WHERE daysdiff > 120</code>

Executing this query will result in an error: "Invalid column name daysdiff". This is because column aliases like daysdiff usually cannot be accessed directly in the WHERE clause.

To solve this problem, we can force SQL to execute the SELECT statement first and then the WHERE clause. This can be achieved by using parentheses or common table expressions (CTE).

Method 1: Use brackets

In the modified query below, the SELECT statement is enclosed in parentheses, forcing it to execute before the WHERE clause:

<code class="language-sql">SELECT
   *
FROM
(
   SELECT
      logcount, logUserID, maxlogtm,
      DATEDIFF(day, maxlogtm, GETDATE()) AS daysdiff
   FROM statslogsummary   
) as innerTable
WHERE daysdiff > 120</code>

Method 2: Use CTE

Alternatively, you can use CTE as in the example below:

<code class="language-sql">WITH CTE AS (
   SELECT
      logcount, logUserID, maxlogtm,
      DATEDIFF(day, maxlogtm, GETDATE()) AS daysdiff
   FROM statslogsummary   
)

SELECT
   *
FROM CTE
WHERE daysdiff > 120</code>

By using parentheses or CTEs, you can control the logical order of SQL operations, allowing you to reference column aliases in WHERE clauses and resolve such errors.

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