On: SQL nulls are weird

[wodenokoto]

There are two values, TRUE and FALSE. Null is not a value, it the the lack of a value.

You have a list of people and you ask if they own a car. You didn't get around to asking George, so that, somehow means he owns a car because you are using boolean logic? Or does it mean he doesn't own a car, because you are using boolean logic?

No, it means you haven't gathered this data point and don't know.

[bunderbunder → wodenokoto]

If there are exactly two possible values, TRUE and FALSE, you're working with Boolean logic.

If there are three possible values, TRUE, FALSE and NULL (unknown), then you're probably working with something like Kleene logic. You can't truly be working with Boolean logic, though, any more than you can be doing integer arithmetic when 15.37 is a member of your domain.

To put it another way, if we're talking about the mathematical definition of boolean algebra and not just some programming language's quirky implementation that happens to be called "bool", then boolean values would by definition be non-nullable. That logic that allows nulls has been implemented using the same unified set of keywords and operator names is a pragmatic decision that simplifies the language implementation and spec, not a principled one that tries to be pedantic about mathematical terminology.

[mrkeen → bunderbunder]

> That logic that allows nulls has been implemented using the same unified set of keywords and operator names is a pragmatic decision

That's why it's name-squatting. Rather than introduce a 'kleene' datatype & operations, and let the user opt-in, they decided that in our 'bool' world, the following is not a truism:

  a = a or a <> a

[mrkeen → wodenokoto]

> No, it means you haven't gathered this data point and don't know.

This is how it should be.

> Somehow means he owns a car because you are using boolean logic?

This is how it unfortunately is. There are 3 people, and there are 3 people who don't have a NULL car. Therefore George has a car.

  CREATE TABLE people(name text, carId uuid);
  INSERT INTO people values('Bill', '40c8a2d7-1eb9-40a9-b064-da358d6cee2b');
  INSERT INTO people values('Fred', '3446364a-e4a5-400f-bb67-cbcac5dc2254');
  INSERT INTO people values('George', NULL);

  SELECT Count(*) FROM people WHERE name NOT IN (
    SELECT name FROM people WHERE carId = NULL
  );

Elsewhere people have argued that NULL propagates, so that your small unknowns infect larger queries. I could get behind that line of thinking, but the query above confidently returns 3.

[mrighele → mrkeen]

> There are 3 people, and there are 3 people who don't have a NULL car.

This is not what you are asking with your query: as someone else stated, NULL is meant to be "UNKNOWN", or "it could be any valid value".

So nothing is ever equal to something that can be anything, because even another NULL (i.e. unknown) value is in general different.

So in the line

    SELECT name FROM people WHERE carId = NULL

the condition will always be false. Now if instead if meant to search for the rows where carId is actually unknown you have to write

    SELECT name FROM people WHERE carId is NULL

And your query will return as one may expect 2.

[mrkeen → mrighele]

> So nothing is ever equal to something that can be anything

It's worse than that. It's something is neither equal nor not-equal to something else.

Whether you can implement something differently as a workaround is immaterial. It's weird, per the title.