Re: c.d.theory glossary - NULL

On Tue, 27 Apr 2004 13:02:56 -0400, "Laconic2" <laconic2_at_comcast.net> wrote:

>> But those "nulls" are very different to the SQL nulls. SQL nulls are
>> logical, not physical.
>
>Your statement is implementation dependent.
>
>InOracle, a zero byte in a VARCHAR field indicates a NULL. It also
>indicates a character string of length zero. That's why you can't tell them
>apart.

The zero byte is at the physical level and the null at the logical level.

>In DEC Rdb, every row is stored in a "record". (The user never sees this).
>At the end of every record is a bit vector marking "missing data". That's a
>physical structure. And it indicates precisely the same thing that the SQL
>NULL indicates.

The bit vector is at the physical level and the SQL nulls at the logical level.

Regards
  Alfredo Received on Tue Apr 27 2004 - 22:47:22 CEST