[zigzag312]

Thanks. It seems that the issue arises from value equality vs optional value state equality.

To me, a more natural way to treat NULLs is to think of NULL not as a value, but as a state. Several customers with a street=null all have street property in an equal state. However, an equal state doesn't mean the value is also equal, as there is no value in that state. Option type in functional languages models this perfectly:

  'a option = 
    |None       // no value here
    |Some of 'a // value of type 'a

So, when checking, if customers have the same street we need to check the value. So, comparision is only valid if street has value defined. Adding null check in addition to equality check to ensure that the actual values are equal feels most natural to me (as this is how it's done in most languages).

Unique constraints should not be a problem. Unique constraints are about values. So if state is NULL there is no value and so constraint does not treat NULL states as equal values. Adding a keyword to change constraint behavior when needed, would be best IMO, as it would need to be rarely used.

[masklinn]

> So, when checking, if customers have the same street we need to check the value. So, comparision is only valid if street has value defined. Adding null check in addition to equality check to ensure that the actual values are equal feels most natural to me (as this is how it's done in most languages).

Is it tho?

Languages with ubiquitous nullability will accept both being null or both being non-null and the same value. Languages with option types do the same (as long as the value is equatable obviously), this includes OCaml: https://v2.ocaml.org/api/Option.html#preds.

The only languages I can think of which seriously diverge would be C and C++ where this is UB.

Maybe if you way stretch it Java because it doesn't have operator overloading, but even there there's Object.equals which is null-safe... and returns `true` if both parameters are `null`.

[zigzag312]

I completely agree. I tried to explain basically the same thing as you, but it seems there is some ambiguity in my example I didn't intend.

Comparing two nullable/option values when both are in a "null state" will return true in most languages. Which, to me, is the most logical result as they have the same state. So, if you want to check if two people actually live on the same street, it's not enough to check p1.street == p2.street, but you need to also check that at least one of the streets is not null. Two nulls would return true, but if equality returns true and we know that one of the streets isn't null, we know we are comparing actual street values (not null states). Sadly, in SQL, NULL=NULL result is not true, but NULL.