conversion from string to decimal isn´t working (or i´m doing it wrong)

CREATE TABLE `tmp` (
`id` int(10) unsigned NOT NULL DEFAULT '0',
`datahora` decimal(17,6) NOT NULL DEFAULT '0.000000',
`value` decimal(20,10) NOT NULL DEFAULT '0.0000000000',
PRIMARY KEY (`id`,`datahora`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1

====
INSERT INTO `tmp` (`id`, `datahora`, `value`) VALUES (1, 1340977500.000000, 1.0000000000);
====

this don´t work...:
SELECT * FROM tmp WHERE id=1 AND
datahora="1340977500.00000000000000"

this work: (without conversion)
SELECT * FROM tmp WHERE id=1 AND
datahora=1340977500.00000000000000

any idea?