Re: varchar value comparisons not working?

Shawn Tayler <stayler@washoecounty.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