Thanks Aleksey for workaround.

I uses CONVERT(,) function, but NULLIF() is datatype invariant.

I check various function, and :

SELECT f1+0 f1 FROM AR
is local nullable

but:

SELECT ISNULL(f1,NULL) f1 FROM AR

is not nullable on local cursor.

SQL optimizator resolve null invariance of not nullable field.
Remarkable!

I hope on VFP get this capability on next release.

Example:
CURSORSETPROP('Nullable','fieldName'[, cTableAlias | nWorkArea ])

CURSORSETPROP('NotNullable','fieldName'[, cTableAlias | nWorkArea ])

Fabio