I've written an IF such that...

IF (SELECT SUM(a.balance + a.overdraft) AS "Funds Available"
FROM Account a
WHERE a.accountID = "12345") < 0
THEN
ROLLBACK
ELSE
COMMIT
ENDIF;

But this is wrong and I can't find any comprehensible details online. Can anyone explain how I'm supposed to structure this?

is the manual any use?
just a minor point.. you are using INNODB tables I presume

Yeah, I'd looked at that.

Solved it now. Basically, the code was correct but it has to be in a procedure or function for it to work which wasn't explicitly stated. I had assumed it could be in the general code.