sql语句中的变量使查询挂起 [英] Variables in sql statement make query hang

问题描述

以下sql语句可提供应有的功能.在定义的时间范围内的最小值.

the following sql statement delivers what it should. The min value in a defined timeframe.

select value, datetime from Schuppen 
where (value = (select min(value) from Schuppen 
    where (measure = 'temp') 
    and datetime between '2018-11-01 00:00:00' and '2018-11-02 00:00:00')) 
and datetime between '2018-11-01 00:00:00' and '2018-11-02 00:00:00';

当我使用变量而不是硬编码日期时，该语句将挂起.

When I use a variable, instead of the hardcoded dates, the statement hangs.

set @startdate = cast('2018-11-01 00:00:00' as datetime);   
select value, datetime from Schuppen 
where (value = (select min(value) from Schuppen 
    where (measure = 'temp') 
    and datetime between @startdate and '2018-11-02 00:00:00')) 
and datetime between '2018-11-01 00:00:00' and '2018-11-02 00:00:00';

我想知道为什么此声明不适用于我的maria db版本10.1.26-MariaDB-0 + deb9u1.

I wonder why this statement does not work against my maria db version 10.1.26-MariaDB-0+deb9u1.

通过mysql控制台尝试.

Tried via the mysql console.

mysql --user=XXXX --password=XXXX Outdoor-Air
...
Your MariaDB connection id is 194
Server version: 10.1.26-MariaDB-0+deb9u1 Debian 9.1

...

MariaDB [Outdoor-Air]> set @startdate = cast('2018-11-01 00:00:00' as datetime);
Query OK, 0 rows affected (0.00 sec)
MariaDB [Outdoor-Air]> select @startdate;
+---------------------+
| @startdate          |
+---------------------+
| 2018-11-01 00:00:00 |
+---------------------+
1 row in set (0.01 sec)

MariaDB [Outdoor-Air]> select value, datetime from Schuppen where (value = (select min(value) from Schuppen where (measure = 'temp') and datetime between @startdate and '2018-11-02 00:00:00')) and datetime between '2018-11-01 00:00:00' and '2018-11-02 00:00:00';

现在语句挂起.

推荐答案

SELECT  `value`, `datetime`
    FROM  Schuppen
    WHERE  measure = 'temp'
      AND  `datetime` >= '2018-11-01'
      AND  `datetime` <  '2018-11-01' + INTERVAL 1 DAY
    ORDER BY `value`  ASC
    LIMIT  1;

这应该有所帮助:

INDEX(measure, datetime, value)

此公式与您的公式之间可能存在的差异:如果最低值在当天发生多次，则只会显示一行.

One potential difference between this formulation and yours: This will show only one row if the lowest value occurs more than once on that day.

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