SYSCOLUMNS: Nullable vs Is_Nullable

[HankFay]

The SYSCOLUMNS doc https://www.lianja.com/doc/index.php/SYSCOLUMNS has the following 2 definitions listed:



and



What's the difference?

I can imagine a difference: in VFP there is no such thing as a NULL value of a table field. It can be empty, with a system-supplied default value, but it will never be NULL. So one of these might be that, and the other might be "this field cannot be set to NULL, and the record will not SAVE if it is."

I understand the NULL and NOT NULL SQL Constraints: On and Off flags. I don't understand these two.

Additionally, Nullable is list as N (2). I am puzzled.

thanks,

Hank

[barrymavin]

Hi Hank,

Lianja maintains a bit array for each row that denotes NULLs for the columns. This bit array is part of the row.

I will check the code and let you know the difference if any. It may just be a legacy thing that was left in.

[HankFay]

Thanks.

It gets even more interesting: SYSCOLUMNCONSTRAINTS also has two different NULL-related attributes:

• NOT NULL
• NULL SUPPORT

The NULL SUPPORT does support the perspective of whether a Field can ever be a NULL, whether or not it's allowed to be NULL when a SAVE occurs.

Hank

[barrymavin]

Hi Hank,

Please see this doc.
https://www.lianja.com/doc/index.php/SET_NULL

With SET NULL OFF the table cannot contain NULL values.

Then the column constraint NOT NULL can override.