it's not possible directly using alter table, your options are:

alter table add newcol
update set newcol = oldcol
drop oldcol, etc

there is an update to the system tables, posted by Ibrahim Bulut

NOT NULL FIELD ==> NULLABLE FIELD
================================
UPDATE RDB$RELATION_FIELDS
SET
RDB$NULL_FLAG=NULL
WHERE
(RDB$FIELD_NAME = 'FIELD_NAME') AND
(RDB$RELATION_NAME='TABLE_NAME');

DELETE FROM RDB$RELATION_CONSTRAINTS
WHERE
(RDB$CONSTRAINT_TYPE = 'NOT NULL') AND
(RDB$RELATION_NAME = 'TABLE_NAME') AND
(RDB$CONSTRAINT_NAME = 'INTEG_7');

>From: "skotaylor" <scott@...>

>I'm want to alter a column from 'NOT NULL' to 'NULL', it doesn't need
>to be 'NOT NULL' it's not part of any constraints. I'm having a hard
>time with the syntax, or is it even possible?

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