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.
