Strange oracle SQL results, using NULL and <>

here are a couple of SQL examples, test your skills.

back ground:

Select count(1) from CRPDTA.F03B14;

>20467464 rows

select count(1) from CRPDTA.F03B14 WHERE rzpost <> ‘D’

>0 rows

select count(1) from CRPDTA.F03B14 WHERE RZPOST = ‘D’

>2767142 rows

So, do you see the problem?  2.7 million rows are ‘D’, 0 are not ‘D’, yet there are 18 millions rows not accounted for!

These rows ARE NULL!

So, what would I get with:

select count(1) from CRPDTA.F03B14 where RZPOST = NULL ?

>0 rows

WTF???  0, I thought I’d get 18000000.  Well, here is another idiosyncrasy of oracle SQL:

select count(1) from CRPDTA.F03B14 where RZPOST IS NULL;

>17709320 rows