I'm trying to write a SELECT query that would add different conditions based on the existence of a connection-specific variable. If that variable exists, its value should be used, otherwise a different value. So I would like to use the VAREXISTS test within the same statement the variable's value is used:

-- optional varibale creation:
-- create variable varTest int = 1;

select varexists('varTest'),
   if varexists('varTest') = 1 then varTest else 'Value unknown' end if;

However, this returns SQLCODE -143 (Column 'varTest' not found) in case the variable does not exist, so it seems to be invalid to use the variable within the same statement as the VAREXISTS test.

In contrast, the following does work - but requires two distinct SELECT queries:

if varexists('varTest') = 1 then
   select varTest;
else
   select 'Value unknown';
end if;

My attempt with the combination of testing and using the variable in one statement may be influenced by C-based short circuit tests, such as "if (pTest && pTest->SomeFunction())" because the language guarantees that the second condition will only be tested if the first condition is true. Of course, SQL has different rules for conditions, and there's no "short circuit logic" there so I do assume a statement refering to an non-existing column/variable must fail.

Is my assumption correct?