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            WITH
  CTE AS (
    SELECT
      files.idFile,
      files.dateAdded,
      ROW_NUMBER() OVER (
        ORDER BY
          files.strFilename ASC
      ) AS rn
    FROM
      files
      INNER JOIN movie ON files.idFile = movie.idFile
  )
UPDATE files
JOIN CTE ON files.idFile = CTE.idFile
SET
  files.dateAdded = DATE_ADD('2024-03-13 08:00:00', INTERVAL CTE.rn SECOND);
          

Fixes:

• Changed rn = ROW_NUMBER() to ROW_NUMBER() OVER (...) AS rn for correct syntax in the CTE.
• Specified the columns files.idFile and files.dateAdded in the CTE selection to avoid ambiguity and because * is not necessary for the operation.
• Replaced UPDATE CTE with UPDATE files JOIN CTE ON files.idFile = CTE.idFile because you cannot directly update a CTE. Instead, you join the CTE with the original table and update the table.
• Changed DATEADD to DATE_ADD and adjusted its syntax to match MariaDB's requirements, as DATEADD is not a function in MariaDB.