I'd vote for just ripping out the:
| A third state, "unknown", is represented by the SQL null value.
sentence entirely. I see no reason why NULL should be talked about in
particular on the page about boolean data types; there are many data
types, any of which might be NULL.
NULL is not unique to boolean, but UNKNOWN is - it would surely be wrong
to have no mention of it at all on this page. This is because the
boolean type is the only one used to represent truth (or logical)
values. One of the comments from the link you provided:
What’s even more interesting is that for BOOLEAN they invented the
keyword UNKNOWN and the 2003 standard states “The null value of the
boolean data type is equivalent to the Unknown truth value.” So for
BOOLEAN (and only BOOLEAN AFAICT) you’re supposed to say WHERE
<boolean primary> IS [NOT] UNKNOWN. And in the definition of
“literal”, which is supposed to “Specify a non-null value”, “boolean
literal” is equated to TRUE, FALSE or UNKNOWN (but the latter is
equivalent to a “null value” a few pages later).
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