Re: c.d.theory glossary - NULL

> 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.

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. Received on Tue Apr 27 2004 - 19:02:56 CEST