Suppose I have a simple boolean expression involving only constants, like

4 + 5 = 9

Maybe (for debugging purposes - maybe the expression is a complicated LIKE expression) I want to just check to see if this expression is true or false. Is there a simple trick for doing this?

Currently I'm just using

SELECT * FROM my_schema.my_table WHERE 4 + 5 = 9;

and seeing if I get anything back. I thought SELECT 1 WHERE expression might work, but no luck (MySQL 5.6.10). To be more precise about what I mean by "simple trick", let's say we want a command which will run without modification on any server. My current command doesn't fit the bill since we need to know the name of a schema and table in the server.


1 Answer 1


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)
         ELSE            'UNKNOWN'
       END AS expression_result ;


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).

  • 1
    So to be clear, expression is just a variable name that can be anything, like result or value.
    – Jack M
    Sep 19, 2019 at 8:18
  • Yes, it's just an alias/name for the column in the result set. You can pick any other name you like. Sep 19, 2019 at 8:20
  • 1
    @JackM To obtain verbal result instead of number one you may use SELECT ELT(COALESCE(1+(4 + 5 = 9),3),'FALSE','TRUE','UNKNOWN');.
    – Akina
    Sep 19, 2019 at 8:22
  • @Akina right, thnx! I added an alternative for that (prefering CASE to the cryptic ELT ;) Sep 19, 2019 at 8:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.