Uma abordagem seria usar uma união:
WITH cte AS (
SELECT "FILE", ID, PARENTID, SHOWCHILD, CAT1, CAT2, CAT3, TOTAL, 1 AS position
FROM yourTable
UNION ALL
SELECT 'Tot', 'Res', 'Res', 'N', SUM(CAT1), SUM(CAT2), SUM(CAT3), SUM(TOTAL), 2
FROM yourTable
WHERE SHOWCHILD = 'Y'
)
SELECT "FILE", ID, PARENTID, SHOWCHILD, CAT1, CAT2, CAT3, TOTAL
FROM cte
ORDER BY
position,
"FILE";