Re: varchar value comparisons not working?

Shawn Tayler <stayler(at)washoecounty(dot)us> writes:
> I run the following:

> select sfd.lid as sflid,sd.lid as slid,sfd.serial from sfd,shawns_data
> sd where sfd.serial = sd.serial_number order by sfd.lid;

> the lid columns in both tables should be identical, but as you see in
> this sample, they do differ:

> sflid | slid | serial
> -------+-------+----------
> 14056 | 14056 | 9614583
> 14057 | | 9614984
> 14058 | 14058 | 9614737
> 14059 | 14059 | 9614579
> 14060 | | 9614827
> 14061 | 14061 | 9614726
> 14062 | 14062 | 9614966
> 14063 | 14063 | 9615079

> So running this query:

> select count(*) from sfd,shawns_data sd where sfd.serial = sd.serial_number and sfd.lid != sd.lid;

> I should show some rows that do not match, at least 2 (there are more than shown).

> But instead I get this:

> count
> -------
> 0
> (1 row)

Probably those "blank" values of slid are really NULLs. A NULL isn't
"equal to" something else, but it isn't "unequal" either. You could
use IS DISTINCT FROM instead of != in your second query.

regards, tom lane