Yeah that's true. All the database can say is "I don't have a value here". Not "there exists no value for this column".
But this doesn't mean that two NULLs are the same. Two people with NULL tax IDs are not the same (one turns out to be unknown but existing, the other is a toddler without a tax ID). Ten people with NULL address don't live in the same house. Half declined to give address, the other half are homeless. Either way we can't send mail to them.
If you use the database to have a UNIQUE constraint of "only one purchase per household" then it makes no sense to allow NULL addresses, but only allow the first NULL address purchaser to buy the thing.
Or "sorry, in this hotel we only allow one guest at a time without a car, and someone else already doesn't have a car".
Does that guest without a car actually have a car? I don't think that's something that the database can solve.
Should databases have two separate values for unknown or no value? That sounds like a world of hurt, with two types of null.
> Should databases have two separate values for unknown or no value? That sounds like a world of hurt, with two types of null.
It’s also a non-problem in practice, because if an application needs to distinguish between multiple types of nulls, it can very easily just use an extra column holding the information needed to disambiguate.
Similarly, the need for non-boolean columns is a non-problem in practice, because an application can just make an extra column for every bit it needs to store. If it needs strings or arbitrary-width numbers it can just join on other tables that store those bits.
(Existence of a workaround doesn't turn a problem into a non-problem)
I stand by it being a non-problem in practice because the workaround is trivial, and the supposed solution adds complexity everywhere.
There isn’t even a universally agreed methodology for how to handle nulls in the relational model in theory.
It’s a semantic problem that can only be solved with reference to the requirements of a particular use case, and when you do that using explicit extra columns, the problem evaporates.
Which happen in world of javascript. It has null and undefined. Null variable is a variable / property that's defined and set with null, while undefined is a variable / property that's not defined yet (or maybe not initialized, I forgot).
I hope at least SQL will give another comparison operation, such as the current equal sign (=) means a.prop is not null && b.prop is not null && a.prop equal b.prop (for not defined yet).
Let's say another sign like a.prop *= b.prop, meaning a.prop is null && b.prop is null || a.prop = b.prop (for our intention is both value are empty).
But this doesn't mean that two NULLs are the same. Two people with NULL tax IDs are not the same (one turns out to be unknown but existing, the other is a toddler without a tax ID). Ten people with NULL address don't live in the same house. Half declined to give address, the other half are homeless. Either way we can't send mail to them.
If you use the database to have a UNIQUE constraint of "only one purchase per household" then it makes no sense to allow NULL addresses, but only allow the first NULL address purchaser to buy the thing.
Or "sorry, in this hotel we only allow one guest at a time without a car, and someone else already doesn't have a car".
Does that guest without a car actually have a car? I don't think that's something that the database can solve.
Should databases have two separate values for unknown or no value? That sounds like a world of hurt, with two types of null.