CREATE TABLE IF NOT EXISTS `_num_qty` (
  `id` int(11) NOT NULL,
  `storage_1` decimal(10,3) NOT NULL DEFAULT '0.000',
  `reserve_1` decimal(10,3) NOT NULL DEFAULT '0.000',
  `storage_2` decimal(10,3) NOT NULL DEFAULT '0.000',
  `reserve_2` decimal(10,3) NOT NULL DEFAULT '0.000',
  KEY `id` (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

INSERT INTO `_num_qty` (`id`, `storage_1`, `reserve_1`, `storage_2`, `reserve_2`) VALUES
(2500011, '5.000', '0.000', '0.000', '0.000'),
(2500026, '30.000', '0.000', '10.000', '0.000'),
(2500030, '0.000', '0.000', '0.000', '0.000');

SELECT q.* FROM _num_qty AS q 
WHERE (q.id='2500026' AND (q.storage_1-50)<0) 
   OR (q.id='2500011' AND (q.storage_1-6)<0)

SELECT q.*,q.storage_1-X AS rest FROM _num_qty AS q