The document you link uses unknown as a synonym for null. If you inserted the result of such a comparison into a table, the value inserted would be NULL.

I'm self-taught, could you show how to insert the result of such a comparison into a table in mssql? I can only think of using CASE or similar, which would mean no direct conversion.

insert into table (c) values (null != null)

Can you provide a complete working example? I'm just getting "incorrect syntax near !" with both mssql and sqlanywhere.

    create table #nulltest (a bit null);
    insert into #nulltest(a) values (0),(1);
    insert into #nulltest(a) values (null != null);

Some SQL dialects don't recognize != and use <> instead

True, I'll blame vacation mode for not catching that one.

However, that doesn't change things, I can't get any of these to parse:

    insert into #nulltest(a) values (1 <> 0);
    insert into #nulltest(a) values (null <> null);
    insert into #nulltest(a) values (true and true);
    insert into #nulltest(a) values (null and null);

So my question remains, how do you insert an UNKNOWN?

The point of me asking is that it was my belief contrary to what was suggested, UNKNOWN is not the same as NULL and, crucially, does not evaluate as NULL.

It's good to see that everyone agrees on the definition of NULL.

Yes: a billion-dollar mistake.

This is s misunderstandig of Hoares “billion dollar mistake”. The mistake was not the existence of nulls, the mistake was type systems where every reference could be null.

Sql does not have this problem.

SQL absolutely does have that problem. You have no way of enforcing or checking that an expression is supposed to be non-NULL; at best you get an error at runtime when you try to insert it into a given column, which is usually far too late.

This is only a problem insofar as any data constraint is a problem.

No, it's a specific problem of SQL that you can apply constraints to stored or quasistored columns but not to expressions. In a real typed language every expression has a type.

And with RDBMSs, a multi-trillion dollar success!