[reshlo]

UNDEFINED would not be accurate. If your signup form has an optional field for a full name which I don’t fill in, I still have a name. Just because a value is not known by your database doesn’t mean it isn’t defined.

E. F. Codd thought about this issue.[0]

> Codd indicated in his 1990 book The Relational Model for Database Management, Version 2 that the single Null mandated by the SQL standard was inadequate, and should be replaced by two separate Null-type markers to indicate why data is missing. In Codd's book, these two Null-type markers are referred to as 'A-Values' and 'I-Values', representing 'Missing But Applicable' and 'Missing But Inapplicable', respectively. Codd's recommendation would have required SQL's logic system be expanded to accommodate a four-valued logic system. Because of this additional complexity, the idea of multiple Nulls with different definitions has not gained widespread acceptance in the database practitioners' domain. It remains an active field of research though, with numerous papers still being published.

[0] https://en.wikipedia.org/wiki/Null_(SQL)

[masklinn]

> UNDEFINED would not be accurate. If your signup form has an optional field for a full name which I don’t fill in, I still have a name.

If your signup form has an optional field for middle name which I don’t fill, it can absolutely be because I don’t have a middle name. It’s undefined and known to be so.

> E. F. Codd thought about this issue.[0]

And because four value logic was left out, nulls have to fulfil multiple incompatible roles, and ends up being weird.

[reshlo]

> If your signup form has an optional field for middle name which I don’t fill, it can absolutely be because I don’t have a middle name.

The fact that some NULLs are indeed undefined doesn’t contradict my point that it would be inaccurate to refer to all NULLs as UNDEFINED.

> It’s undefined and known to be so… And because four value logic was left out, nulls have to fulfil multiple incompatible roles, and ends up being weird.

You know it to be undefined, but if you just leave that field blank on the form, the database still can’t know whether that’s because you don’t have one or because you didn’t provide the one you have, unless there was a field on the form that allowed you to explicitly indicate that you don’t have one.

[benzayb]

> If your signup form has an optional field for a full name which I don’t fill in, I still have a name. Just because a value is not known by your database doesn’t mean it isn’t defined.

There's the Closed World Assumption in a database.

[reshlo]

It’s often a bad idea to make that assumption.