Detecting UNKNOWN truth values using IIF or some other means

Question

Hi,
I'm researching how to handle UNKNOWN truth value in my CHECK constraints, but I just want to know in general how to tell if an arbitrary boolean expression is TRUE, FALSE, or UNKNOWN, without having to specially rewrite the boolean condition to mean unknown.
The IIF() function returns the 2nd argument if the 1st argument is TRUE, and the 3rd argument if the 1st argument is FALSE or UNKNOWN.  So the only way I can think of to specifically find the UNKNOWN ones is to try IIF() with <boolean_expression>
and NOT(<boolean_expression>).  If it takes the 3rd argument both times, that means it's UNKNOWN.
Is there a better way
-- Returns 0
SELECT CASE WHEN NULLIF(IIF(1 < 2, 'T', 'F') + IIF(NOT(1 < 2), 'T', 'F'), 'FF') IS NULL THEN 1 ELSE 0 END IS_UNKNOWN
-- Returns 1
SELECT CASE WHEN NULLIF(IIF(NULL < 2, 'T', 'F') + IIF(NOT(NULL < 2), 'T', 'F'), 'FF') IS NULL THEN 1 ELSE 0 END IS_UNKNOWN
I'd want to replace an arbitrary boolean expression in the formula and just out if it is UNKNOWN.

Answer 1

this?
DECLARE @Param1 int=2
DECLARE @Param2 int = 2
SELECT IIF(@Param1 IS NULL OR @Param2 IS NULL,'Unknown',IIF(@Param1<@Param2,'True','False'))
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