> Some DB's say that an empty string is the same as null, it doesn't mean
> they're right. In fact, it can be rather inconvenient if an empty string
> in your data also has a meaning (namely 'known to be an empty string'
> instead of 'unknown')!
This is the behavior in Oracle. And I found that out the hard way...
what a PITA.
Anyhow, this isn't specific to Postgres, but here's a Wikipedia entry
for how ternary or 3 valued logic works.
http://en.wikipedia.org/wiki/Ternary_logic