EventProcessor.getInstance().addListener(new EventListener()
    {
      public void notify(ContractBalanceChangedEvent e, EventListenerContext ctx)
        throws BGException
      {
        //тут делаем свои грязные дела
      }
    }
    , ContractBalanceChangedEvent.class);

SELECT contract.id, (SELECT contract_balance.summa1+contract_balance.summa2-contract_balance.summa3-contract_balance.summa4 FROM contract_balance WHERE contract_balance.cid=contract.id AND ((contract_balance.yy*12) + contract_balance.mm)<=((YEAR(CURDATE())*12) + MONTH(CURDATE())) ORDER BY yy DESC, mm DESC LIMIT 1) as 'balance' FROM contract
WHERE (SELECT contract_balance.summa1+contract_balance.summa2-contract_balance.summa3-contract_balance.summa4 FROM contract_balance WHERE contract_balance.cid=contract.id AND ((contract_balance.yy*12) + contract_balance.mm)<=((YEAR(CURDATE())*12) + MONTH(CURDATE())) ORDER BY yy DESC, mm DESC LIMIT 1) > 0

 CREATE FUNCTION `get_balance`(contract int(10) unsigned) RETURNS decimal(10,2)
    READS SQL DATA
    COMMENT 'returns current balance by cid'
BEGIN DECLARE balance decimal(10,2);  SELECT summa1+summa2-summa3-summa4 INTO balance FROM contract_balance where cid=contract and (yy<<4)+mm<=(year(now())<<4)+month(now()) ORDER BY yy desc, mm desc LIMIT 0,1; RETURN balance; END

(yy<<4)+mm<=(year(now())<<4)+month(now())

SELECT t1.id, t1.balance
FROM (
SELECT contract.id, (SELECT contract_balance.summa1+contract_balance.summa2-contract_balance.summa3-contract_balance.summa4 FROM contract_balance WHERE contract_balance.cid=contract.id AND (contract_balance.yy<<4)+contract_balance.mm<=(year(now())<<4)+month(now()) ORDER BY yy DESC, mm DESC LIMIT 1) as 'balance' FROM contract
) t1
WHERE t1.balance >0

SELECT contract.id, contract_balance.summa1+contract_balance.summa2-contract_balance.summa3-contract_balance.summa4 as balance
FROM contract
  LEFT JOIN contract_balance
  ON (contract.id=contract_balance.cid AND contract_balance.mm=month(now()) AND contract_balance.yy=year(now()))

select * from contract_balance t1
inner join (select cid, max(yy*12+mm) m from contract_balance group by cid) t2 on (t1.cid=t2.cid AND (t1.yy*12+t1.mm)=t2.m)

SELECT SQL_NO_CACHE
   *
FROM
   contract_balance t1
   INNER JOIN (
      SELECT
         cid, MAX(100*yy + mm) m
      FROM
         contract_balance
      GROUP BY cid
   ) t2 ON t1.cid = t2.cid AND (100*t1.yy + t1.mm) = t2.m;
-- 0.312 ms

SELECT SQL_NO_CACHE
   cid, SUM(summa2 - summa3 - summa4) AS balance
FROM
   contract_balance t1
GROUP BY
   cid;
-- 0.063 ms