I need to calculate currency* debit - currency* credit = running balance in mysql

See more:

PHP

MySQL

HTML5

table 1 = id, name, ACCOUNTID, detail, currency, rate, credit,
1, jhon, 1 . test , USD, 3.37, 10000,

table 2 = id, name, ACCOUNTID, detail, currency, rate, DEBIT,
1, jhon, 1 test , USD, 3.37, 5000,

table 3 = id, name, ACCOUNTID,
1, jhon, 1

i need running balance ?

What I have tried:

SELECT j.id,j.kata_id,j.credit,j.buyRate,
       SUM(j.credit*j.buyRate) tCredit,
       SUM(b.debit*b.sellRate) tdebit,
       SUM((j.credit*j.buyRate)-(b.debit*b.sellRate ))  Mawjodi
FROM tbl_roznamcha_jam j
JOIN tbl_roznamcha_banam b ON j.kata_id = b.kata_id
GROUP BY j.id
ORDER BY j.id