MySQL, how to make negative results possible when subtracting unsigned values?

Question

I have three tables joined by a left join. Here is the code:

SELECT (LEAST(`a`.`price, `b`.`price`) - `c`.`price`) AS `diff` ... ORDER BY `diff` DESC 

Problem: c . price greater than LEAST, so the subtraction is negative and throws the BIGINT UNSIGNED value out of range. How can I make NOT throw this ridiculous mistake?
This is the result data, I do not change the actual data in the table, so why does this not allow me to do this normally? I tried CAST(LEAST(...) AS SIGNED) and discarded both columns inside LEAST as signed, did not work.

Answer 1

SIGNAL each number before LEAST and before subtraction

 SELECT (LEAST(CAST(`a`.`price` AS SIGNED), CAST(`b`.`price` AS SIGNED)) - CAST(`c`.`price` AS SIGNED)) AS `diff` ... ORDER BY `diff` DESC 

Answer 2

You can check the NO_UNSIGNED_SUBTRACTION : http://dev.mysql.com/doc/refman/5.0/en/server-sql-mode.html#sqlmode_no_unsigned_subtraction .

There are risks when using: http://datacharmer.blogspot.fi/2006/11/hidden-risks-of-sql-mode.html