chyba ve výpočtu sum při left join

SELECT date, SUM((prodej*((100-sleva)/100)-nakup)*(100/(100+dph))*(ks_prodej-ks)) zisk FROM (SELECT * FROM deni_prehled UNION ALL SELECT * FROM zaloha_deni_prehled) deni_prehled_virtual left join vracene_zbozi on (deni_prehled_virtual.date=vracene_zbozi.datum_prodeje AND deni_prehled_virtual.autoID=vracene_zbozi.dokladID AND deni_prehled_virtual.produktID=vracene_zbozi.produktID) WHERE autoID>0 AND date!="0000-00-00" GROUP BY YEAR(date),MONTH(date) ORDER BY date

CREATE TABLE IF NOT EXISTS `vracene_zbozi` (
  `ID` int(10) NOT NULL,
  `dokladID` int(10) NOT NULL,
  `produktID` int(10) NOT NULL,
  `datum_vraceni` date NOT NULL,
  `datum_prodeje` date NOT NULL,
  `ks` int(10) NOT NULL DEFAULT '0'
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

CREATE TABLE IF NOT EXISTS `deni_prehled` (
  `ks_prodej` int(10) DEFAULT NULL,
  `sleva` int(10) DEFAULT NULL,
  `autoID` int(10) NOT NULL,
  `produktID` int(11) NOT NULL,
  `date` date NOT NULL,
  `nakup` decimal(7,2) NOT NULL,
  `prodej` decimal(7,2) NOT NULL,
  `dph` int(10) NOT NULL,
  `vraceno` int(10) NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8;

CREATE TABLE IF NOT EXISTS `zaloha_deni_prehled` (
  `ks_prodej` int(10) DEFAULT NULL,
  `sleva` int(10) DEFAULT NULL,
  `autoID` int(10) NOT NULL,
  `produktID` int(11) NOT NULL,
  `date` date NOT NULL,
  `nakup` decimal(7,2) NOT NULL,
  `prodej` decimal(7,2) NOT NULL,
  `dph` int(10) NOT NULL,
  `vraceno` int(10) NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8;