API: Should NOT operators return NULL values?
If you do != 5 in SQL, you'll get all rows that aren't 5, not including null values. This is the same in the API and SearchKit. I wonder if this is the desired behaviour? I would think the assumption would be that if I search for all Contributions where the Contribution Page is != 5, I would get all Contributions except those that have Contribution Page = 5 — but what I actually get is all Contributions that have a Contribution Page, but where that Contribution Page isn't 5.
In other words, I think you'd generally expect that if you search all Contributions where Contribution Page = 5 OR Contribution Page != 5, you'd just get all Contributions, but you only get Contributions with a Contribution Page. Obviously, your expectation might change based on your familiarity with SQL.
This is the same for NOT LIKE, NOT REGEX, and so on.
On the one hand, it's generally better to keep the behaviour of the API operators the same as SQL to avoid confusion. On the other hand, I'm having a hard time imagining a search you might want to make with a NOT that you wouldn't want to include null values, so this seems unhelpful and potentially frustrating. Is this something we should consider changing?