Is it not the NULL_FLAG in the RDB$RELATION_FIELDS table that should be set
to 1?

Also, I set the field in the NULL_FLAG in the RDB$RELATION_FIELDS table to 1
using IB SQL but it didn't create an entry in the RDB$RELATION_CONSTRAINTS
table.

Do you need this entry and should it be automatically created?

Thanks.

> RDB$FIELDS holds all field definitions and has a column NULL_FLAG.

> > Finally, what are the two actions involved to go from NOT NULL to
> NULLable?
>
> When you define a column as NOT NULL, two things will happen:
>
> 1) the NULL_FLAG = 1
> 2) a constraint in RDB$RELATION_CONSTRAINTS is created
>
> 2 -> needs to be dropped
> 1 -> needs to be changed.