check contraints incorrectly reject "null"

Given a table definition like:

create table foo (i integer check (i > 0));

I noticed the following works in Oracle but fails in Postgres:

insert into foo values(null);

I was curious about what the standard might say, and had been
meaning to buy Date's book for some time, so broke down and
did so.

According to Date, a check contraint should fail if the expression
evaluates to false. It appears that Postgres only passes the
check constraint if it evaluates to true. In three-valued logic,
these statements aren't equivalent. He has a paragraph about
nulls and check contraints in chapter 14, I believe, and his
explanation makes it clear that Oracle is right, Postgres wrong.

It's easy to fix by adding a check for null to the constraint,
and afterwards the SQL still works with Oracle, but it's still
a bug...

- Don Baccus, Portland OR <dhogaza(at)pacifier(dot)com>
Nature photos, on-line guides, Pacific Northwest
Rare Bird Alert Service and other goodies at
http://donb.photo.net.