Uses a partition criteria to group the row numbering according to it.
SELECT
ROW_NUMBER() OVER(PARTITION BY DepartmentId ORDER BY DepartmentId ASC) AS RowNumber,
DepartmentId, Fname, LName
FROM Employees
WITH cte AS (
SELECT ProjectID,
ROW_NUMBER() OVER (PARTITION BY ProjectID ORDER BY InsertDate DESC) AS rn
FROM ProjectNotes
)
DELETE FROM cte WHERE rn > 1;
