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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.

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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).

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

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