How can I test a constant boolean expression?
You can use

SELECT (4 + 5 = 9) AS expression_result ;


It will either give you 1 (TRUE), 0 (FALSE) or NULL (UNKNOWN).

If you prefer string output instead of numbers:

SELECT CASE (4 + 5 = 9)
         WHEN TRUE  THEN 'TRUE'
         WHEN FALSE THEN 'FALSE'
         ELSE            'UNKNOWN'
       END AS expression_result ;




Alternatively:

SELECT 1 FROM dual WHERE (4 + 5 = 9) ;


It will give you either 1 (TRUE) or no rows in the results (FALSE or UNKNOWN).

I'd prefer to use the first method as it distinguishes between all three possible results of a boolean expression in SQL (TRUE / FALSE / UNKNOWN).