Question

我有一张简单的桌子 value（int），created（timestamp）

我想做一个返回任意行数的查询，其中包含avg（value）和avg（created）。分组函数是创建的顺序，意味着如果我要求2行，我应该根据创建的第一行获得一个集合。

我有下表：

create table  log(value int,created timestamp);

insert into log values
(1,'2016-01-01 00:00:00'),
(2,'2016-01-01 01:00:00'),
(3,'2016-01-01 02:00:00'),

(4,'2016-01-01 03:00:00'),
(5,'2016-01-01 04:00:00'),
(6,'2016-01-01 05:00:00'),

(7,'2016-01-01 06:00:00'),
(8,'2016-01-01 07:00:00'),
(9,'2016-01-01 08:00:00');

http://sqlfiddle.com/#!9/b9a94

我想要检索3行应该是

2-2016-01-01 01:00:00
5-2016-01-01 04:00:00
8-2016-01-01 07:00:00

可以在不使用java或php处理的情况下在单个查询中执行此操作吗？

Answer 1

例如，当您按3分组时，按创建日期排序。您可以将3更改为要分组的大小。

SELECT avg(sub.value), avg(sub.created)
from (
    SELECT @row_number:=@row_number+1 AS row_number,value, created 
    FROM log, (SELECT @row_number:=0) AS t
    ORDER BY created
) sub
group by floor((sub.row_number-1)/3)

更新

如果您总是希望根据行数划分为3组，您可以执行以下操作：

SELECT avg(sub.value), avg(sub.created)
from (
    SELECT @row_number:=@row_number+1 AS row_number, floor((@row_number-1)/c.num) groupid,value, created 
    FROM log, 
      (SELECT @row_number:=0) AS t, 
      (SELECT ceil(COUNT(*)/3) num FROM log) c
     ORDER BY log.created
) sub
GROUP BY sub.groupid

具有指定行数的mysql avg值

1 个答案:

更新