SQL：如何选择每天一个记录，假设每天包含多个值的MySQL(SQL: How to select

我想选择记录'2013-04-01 00:00:00'到'today' ，但是，每一天都有很多的价值，因为他们节省每次15分钟的值，所以我想只有第一个或最后一个值从每一天。

表模式：

CREATE TABLE IF NOT EXISTS `value_magnitudes` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `value` float DEFAULT NULL,
  `magnitude_id` int(11) DEFAULT NULL,
  `sdi_belongs_id` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
  `reading_date` datetime DEFAULT NULL,
  `created_at` datetime DEFAULT NULL,
  `updated_at` datetime DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=1118402 ;

不良的SQL：

SELECT value FROM `value_magnitudes` WHERE `value_magnitudes`.`reading_date` BETWEEN '2013-04-01 00:00:00' AND '2013-04-02 00:00:00' AND (`value_magnitudes`.magnitude_id = 234) LIMIT 1
SELECT value FROM `value_magnitudes` WHERE `value_magnitudes`.`reading_date` BETWEEN '2013-04-02 00:00:00' AND '2013-04-03 00:00:00' AND (`value_magnitudes`.magnitude_id = 234) LIMIT 1
SELECT value FROM `value_magnitudes` WHERE `value_magnitudes`.`reading_date` BETWEEN '2013-04-03 00:00:00' AND '2013-04-04 00:00:00' AND (`value_magnitudes`.magnitude_id = 234) LIMIT 1
SELECT value FROM `value_magnitudes` WHERE `value_magnitudes`.`reading_date` BETWEEN '2013-04-04 00:00:00' AND '2013-04-05 00:00:00' AND (`value_magnitudes`.magnitude_id = 234) LIMIT 1
SELECT value FROM `value_magnitudes` WHERE `value_magnitudes`.`reading_date` BETWEEN '2013-04-05 00:00:00' AND '2013-04-06 00:00:00' AND (`value_magnitudes`.magnitude_id = 234) LIMIT 1
etc ...

我希望所有在一个可能的话...

非常感谢。

编辑：我的意思是，我每天一查询，但我只是想从一个单一查询reading_date >= '2013-04-01 00:00:00' Ç

EDIT2：我有64260条记录在table.value_magnitudes和excecute和响应该查询，有时超时连接如需要sooooooooo长。

Answer 1:

为使您能做到每次约会的第一个条目

select * from value_magnitudes
where id in 
(
    SELECT min(id)
    FROM value_magnitudes
    WHERE magnitude_id = 234
    and date(reading_date) >= '2013-04-01'
    group by date(reading_date)
)

Answer 2:

select * from value_magnitudes
where id in 
(
   SELECT min(id)
   FROM value_magnitudes
   WHERE `value_magnitudes`.`reading_date` BETWEEN '$from_selected' AND '$to_selected' and (magnitude_id = 234) group by date(reading_date)
)