php - SQL：如何每天选择一条记录，假设每天包含多于1个值MySQL

Question

我想从 to 中选择记录'2013-04-01 00:00:00'，'today'但是每一天都有很多价值，因为它们每 15 分钟保存一个值，所以我只想要每天的第一个或最后一个值。

表架构：

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 ;

错误的 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 ...

如果可能的话，我想合一……

十分感谢。

编辑： 我的意思是，我每天都有一个查询，但我只想从reading_date >= '2013-04-01 00:00:00'c进行一次查询

EDIT2： 我有64,260条记录，table.value_magnitudes执行和响应该查询需要很长时间，有时连接超时。

Answer 1

要获得每个日期的第一个条目，您可以这样做

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)
)