Попробуйте: вы должны разделить CTE запятыми, а затем выбрать
With cte1 as
(
SELECT assignment_number,
assignment_status
FROM per_all_assignments_f
),
cte2 as
(
SELECT assignment_number,
assignement_status
FROM per_all_assignments_f
)
SELECT assignment_number
FROM cte1
WHERE assignment_status = ‘ACTIVE’
UNION -- OR UNION ALL
SELECT assignment_number
FROM per_all_assignments_f
WHERE assignment_status = ‘INACTIVE’
ORDER BY assignment_number