列の値を行番号と同じに設定する方法は?

ROW_NUMBER() を使用して列を直接 UPDATE しようとすると、...

...代わりに、テーブルをそれ自体に INNER JOIN します...

UPDATE
    [test123]
SET
    [row_number] = [x].[rn]
FROM
    [test123]
INNER JOIN
    (
        SELECT
            [test_id],
            ROW_NUMBER() OVER (ORDER BY [test_id]) AS rn
        FROM
            [test123]
    ) AS x
ON 
    [test123].[test_id] = [x].[test_id]