Can you reference a calculated field in the same view that it was calculated?
SELECT
g.niin,
CASE
when (e.total_needs + (m.mdr*6)) is null THEN
g.FUD
ELSE
(g.FUD - (e.total_needs + m.mdr*6))
END CASE
FROM
G072_VIEW g,
EXPRESS e,
MDR_VIEW m
WHERE
g.niin = e.niin AND g.niin = m.niin
SELECT
g.niin,
CASE
when (e.total_needs + (m.mdr*6)) is null THEN
g.FUD
ELSE
(g.FUD - (e.total_needs + m.mdr*6))
END CASE
FROM
G072_VIEW g,
EXPRESS e,
MDR_VIEW m
WHERE
g.niin = e.niin AND g.niin = m.niin