SQL design question: null vs. boolean values

Hi all:

I was wondering if anyone had recommendations for the
following scenarios:

(A)
I have three radio boxes in the user form

field_foo
[]yes
[]no
[]unknown

These are mutually exclusive and user input is always
required.

So in the database, should I have something like:

field_foo char(1) not null check (field_foo in 'y',
'n', 'u')
....OR....
field_foo char(1) check (field_foo in 'y', 'n')

The second choice always implies that NULL means
unknown,
whereas for the first choice, unknown is coded as 'u'.

(B)
In the user form, I have a field like:

field_bar
[] select_me

with ONE choice, which is optional.

Should I code this as:

field_bar char(1) not null check (field_foo in 'y',
'n')
....OR....
field_foo char(1) check (field_foo in 'y')

The second choice always implies that NULL means not
selected whereas whereas for the first choice,
selected is coded
as 'y' and not selected coded as 'n'

Any advice, dear SQL experts ?

Best regards,

--j

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