> that the original one-by-one code was not in fact setting the field
> values (even though no exceptions were being thrown). Each of those
> one-by-one updates should have been wrapped in a transaction, which if
> successful would leave the table between transactions with some records
> having the constrained field set, and some not. Would Firebird object
> to that? If so, that would explain what I was seeing, though not why
> an exception didn't get raised.
>
> Regards,
>
> Robert.
>

Sounds right to me. Alter the table field not null, but this time don't
update the field to any value.
Then try anything you like to update just one row and FB will object about
the constraint.
You must update all rows in one hit when making this DDL change.
Alan