🚀 go-pugleaf

Steve Koterski wrote:
> 
> On Wed, 03 Mar 1999 11:36:50 -0800, Mark Berry <markb@nospam.mcbsys.com>
> wrote:
> 
> 
> That's what I figured the problem was -- before I even got that far down in
> the article. Yes, the methods used in SQL (not just local SQL) are
> different for NULL values. You cannot use the normal comparison operators
> available in local SQL: NOT =, <>, or !=. You have to use the NOT NULL
> predicate.
> 
> For example:
> 
>   SELECT *
>   FROM ContSupp C
>   WHERE (C.AccountNo = "90CF-SYSTEM-REC") AND
>     ((C.RecType != "H") OR (C.RecType IS NULL))

Thanks, Steve, but as Nelson said, it seems to work fine with just 

  NOT C.RecType = "H"

Let's see if I understand this...  The "=" operator will find the "H"
row(s), so NOTting it will return all the other rows, including RecType
of NULL (which is, I assume, how it sees an empty value in a table,
though I didn't think dBASE5 tables supported null values).  

I have to admit it still isn't clear to me why that gives a different
result than 

  C.RecType <> "H"

In that row, we're asking "Is NULL <> 'H'?".  So the result is either
True, or did I read somewhere that any expression with NULL in it
returns NULL?

If you can recommend a good book where I can find an explanation, by all
means...

Thanks,

Mark Berry