Re: SQL design question: null vs. boolean values

Hi:

> > (A)
> > I have three radio boxes in the user form
> >
> > field_foo
> > []yes
> > []no
> > []unknown
> > ...
> > field_foo char(1) not null check
> > (field_foo in 'y', 'n', 'u')
> > ....OR....
> > field_foo char(1) check (field_foo in 'y', 'n')
> >

> Option 1 - the value is known, the user made a
> choice and it was to
> click the "unknown" box. The box could be labelled
> "marmalade" just as easily.

I see what you are saying. It's "known" that the user
actually selected something (the choice they selected
just happened to have a label "unknown").

NULL would be apprpriate if they had selected
nothing at all, right ?

However, if a choice is required (meaning the front
end
html form cannot be submitted without some selection
at least), then couldn't we fold unknown into NULL ?

i.e.,:

user choice
yes -> 'y'
no -> 'n'
unknown -> null

Since it's guaranteed that the user will always
choose something ?

In fact, this is as you say similar to:

user choice
yes -> 'y'
no -> 'n'
marmalade -> null

I ran into another issue while designing my front end
html form.

------------------------------------------
field_foo
[ ] yes
[ ] no

if you answered "yes" in field_foo above, you must
enter detail here

foo_detail
[ ]
-------------------------------------------

This is a little tricky to capture in the database.
The issue
is that the nullability of one column depends AT
RUNTIME
on the value of another column (NOT at design time).

I ended up doing something like

create table xyz
(
field_foo char(1) check (field_foo in 'y', 'n'),
foo_detail varchar(255),
check (
case
when field_foo='y' and foo_detail is null
then false
else true
end
)
);

The constraint uses a case that *requires* some value
foo_detail if field_foo is 'y'.

I don't know whether this is the recommended
way to do the above or I'm making things too
complicated.. Any other opinions/suggestions ?

Best regards,

--j

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