How Can I Use ROW_NUMBER() in a WHERE Clause?

Utilizing ROW_NUMBER() Function in the WHERE Clause: Overcoming Windowed Function Restrictions

Windowed functions, such as ROW_NUMBER(), enable powerful data transformations within result sets. However, their usage is subject to specific constraints, one of which is their exclusion from the WHERE clause.

As exemplified in the query encountered by the user, attempting to use ROW_NUMBER() in the WHERE clause results in the error message: "Windowed functions can only appear in the SELECT or ORDER BY clauses."

To address this limitation, a technique can be employed that involves wrapping the original query in a Common Table Expression (CTE), allowing for the inclusion of windowed functions within the CTE query.

Here's how to implement this solution:

1. Create a CTE: Encapsulate the original query within a CTE, assigning an alias (e.g., MyCte) to it.
2. Include ROW_NUMBER() in the CTE: Within the CTE, include the ROW_NUMBER() function along with other desired columns.
3. Select from the CTE: Query the CTE for the intended output, utilizing the ROW_NUMBER() results in the WHERE clause.

The revised query would look like this:

WITH MyCte AS
(
    SELECT employee_id,
            RowNum = ROW_NUMBER() OVER (ORDER BY employee_id)
    FROM V_EMPLOYEE
    ORDER BY Employee_ID
)
SELECT employee_id
FROM MyCte
WHERE RowNum > 0

By wrapping the original query in a CTE, the windowed function ROW_NUMBER() can now be utilized in the WHERE clause, enabling the filtering of results based on its values.

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