Re: CHECK constraint so that only one is TRUE?

incognit(at)unifiedmind(dot)com (+I) wrote in message news:<ebcd475a(dot)0201121104(dot)62d46853(at)posting(dot)google(dot)com>...
> How would you write a check constraint to ensure that only one column,
> of type boolean, is set to TRUE for a given ID?

Need more info. Are you saying that a row has two or more boolean
columns, and for each row you want to ensure that only one is TRUE?

E.g.,

CREATE TABLE foo (
id integer not null,
is_animal boolean not null,
is_vegetable boolean not null,
is_mineral boolean not null,
check(?));

If this is the case, and you need a generic solution, you'd best write
a function to compute it. There's a boolean expression for this,

((a xor b xor c) or (a xor b) and (b xor c) and (a xor c))

(and there's probably a shorter one) but it's hard to generalize it to
N columns.