From: hsv
Date: October 6 2012 1:51am
Subject: Re: (real) silly question about variables...
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For this,
>>>> 2012/10/04 16:13 +0200, MAS! >>>>
IF(GROUP_CONCAT(DISTINCT secA.sec_code  SEPARATOR '|') is null,                                                                                                            
               IF(GROUP_CONCAT(DISTINCT secB.sec_code  SEPARATOR '|') is null, settore, GROUP_CONCAT(DISTINCT secB.sec_code  SEPARATOR '|')),                                             
                 GROUP_CONCAT(DISTINCT secA.sec_code  SEPARATOR '|') ) as settore,  
<<<<<<<<
you really want

IFnull(GROUP_CONCAT(DISTINCT secA.sec_code  SEPARATOR '|'),
               IFnull(GROUP_CONCAT(DISTINCT secB.sec_code  SEPARATOR '|'), settore)) as settore,  

The operation
If this is not null use this, else use that
is far, far too common in SQL for there not to be a function for it.

As for your original question, note this from the MySQL HTML documentation:


8.4. User-Defined Variables

...
As a general rule, you should never assign a value to a user variable and read the value within the same statement. You might get the results you expect, but this is not guaranteed. The order of evaluation for expressions involving user variables is undefined and may change based on the elements contained within a given statement. In SELECT @a, @a:=@a+1, ..., you might think that MySQL will evaluate @a first and then do an assignment second. However, changing the statement (for example, by adding a GROUP BY, HAVING, or ORDER BY clause) may cause MySQL to select an execution plan with a different order of evaluation.