Re: SQL design question: null vs. boolean values

> 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
> )
> );

A simpler check would be :

CHECK(
(field_foo = 'y' AND foo_detail IS NOT NULL)
OR ( (field_foo = 'n' OR field_foo IS NULL) AND foo_detail IS NULL)
)

Which means " field_foo can be y, n, or NULL, and foo_detail should be
null except if field_foo is 'y' "

Also, IMHO, the Y/N/unknown case should have three values, NULL meaning
'the user has not answered this question'. Because if you insert a blank
row in the table and fill it afterwards, you'll know if it was 'answered
unknown' or 'unanswered'.