Why Can't I Use Window Functions in SQL WHERE Clauses?

Understanding the Limitations of Window Functions in WHERE Clauses

SQL Server prohibits the use of window functions within WHERE clauses. This restriction stems from the fundamental order of query processing and potential ambiguity in result interpretation.

SQL standards dictate that window functions operate after the WHERE, JOIN, GROUP BY, and HAVING clauses have been processed. Therefore, including them in a WHERE clause is logically impossible.

The core issue is the uncertainty that arises from varying predicate evaluation order. Consider a scenario where you want to select rows where a column value exceeds a certain threshold, but only within a subset defined by a window function (e.g., the top N rows). The outcome would depend entirely on whether the filtering condition is applied before or after the window function's calculations.

For example:

SELECT col1
FROM T1
WHERE ROW_NUMBER() OVER (ORDER BY col1) > 3 AND col1 > 'B'

The results would differ depending on the evaluation order. To prevent this ambiguity and guarantee consistent, predictable query results, SQL Server prevents window functions from being used in WHERE clauses. This ensures reliable query processing and avoids unexpected outcomes.

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