mysql variable with #variables for calculated values - mysql

Question

I have a mysql query where I need to calculate values like ROUND(SUM(temp.total_pq),2) multiple times, so I defined variables to avoid repeating them.
But the line 5 in the query returns wrong value in the results. The value for #diff_client_partner_qtty := ROUND((#partner_qtty_all_runs - #client_qtty_all_runs), 2) AS diff_client_partner_qtty is always NULL the first time I run and thereafter always 84.
I asked the in-house DBA and he says I should not use variables in my query like this because the order in which mysql will set values for the variable is not predictable and hence I may get NULL value.
But why? Also can someone please propose then another way whereby I can avoid rewriting ROUND(SUM(temp.total_pq),2) multiple times other than a subquery. I would prefer to avoid a subquery because I think even in its current form query is not that readable.
SELECT temp.dtaccounted AS accounting_period,
#partner_qtty_all_runs := ROUND(SUM(temp.total_pq),2) AS partner_qtty_all_runs,
ROUND(temp.mmq,2) AS mopay_qtty,
#client_qtty_all_runs := ROUND(SUM(temp.total_cq),2) AS client_qtty_all_runs,
#diff_client_partner_qtty := ROUND((#partner_qtty_all_runs - #client_qtty_all_runs), 2) AS diff_client_partner_qtty,
#partner_gtv := ROUND(temp.total_pq_gtv, 2) AS partner_gtv,
#client_gtv := ROUND(temp.total_cq_gtv,2) AS client_gtv,
#diff_client_partner_gtv := ROUND((#partner_gtv - #client_gtv), 2) AS diff_client_partner_gtv,
temp.stariffcurrency AS tariffcurrency,
ROUND(#diff_client_partner_gtv * ffactor, 2) AS diff_client_partner_gtv_eur,
temp.scountry AS country,
temp.spartnererpid AS partner_erp_id,
c.name AS partner_name,
temp.nproducttype AS product,
temp.capping
FROM
(SELECT SUM(npartnerquantity) AS total_pq,
SUM(nmindmaticsquantity) AS mmq,
SUM(nclientquantity) AS total_cq,
SUM(dgrosstariff * npartnerquantity) AS total_pq_gtv,
SUM(dgrosstariff * nclientquantity) AS total_cq_gtv,
nrun,
vb.scountry,
vb.spartnererpid,
dtaccounted,
stariffcurrency,
vb.nproducttype,
cq.bisenabled AS capping
FROM report_table vb,
client_table cq
WHERE vb.accperiod > '2013-12-01'
AND vb.partnerid = cq.partnerid
AND vb.scountry = cq.scountry
AND vb.nproducttype = cq.nproducttype
AND (cq.dtvalidto IS NULL
OR cq.dtvalidto > vb.accperiod)
GROUP BY scountry,
nproducttype,
partnerid,
nrun,
accperiod
) temp,
customer c,
currency_conversion cc
WHERE temp.partnerid = c.erp_id
AND temp.total_pq <> temp.total_cq
AND cc.scurrencyfrom = temp.stariffcurrency
AND cc.scurrencyto = 'EUR'
AND cc.dtrefdate = temp.accperiod
GROUP BY temp.scountry,
temp.partnerid,
c.name,
temp.nproducttype,
temp.accperiod
ORDER BY temp.accperiod,
temp.scountry,
temp.partnerid,
temp.nproducttype,
temp.capping \G;