获取每N组的平均值

Question

我看到这里的一些人遇到了和我现在一样的问题......但没有任何帮助：/

我有一个包含 2 列的表 Test。日期和整数值。 我需要每 N 条记录的平均值。 比如

Date        | Info
2013-01-01  |  210
2013-01-02  |  213
2013-01-03  |  214
2013-01-05  |  197
2013-01-06  |  192

....

让我们假设 N = 3

我需要填充一个表格

Averages
212.3333 (that's the average of the 3 first Info's) (210+213+214)/3
208.0000 (213+214+197)/3
201.0000 (214+197+192)/3

我试过了

delimiter $$
DROP PROCEDURE IF EXISTS calcAvg$$
CREATE PROCEDURE calcAvg()
begin

DECLARE v_finished INTEGER DEFAULT 0;
declare v_avg DECIMAL default 0.0;
DECLARE initial_date date;

DEClARE cr CURSOR FOR 
    select avg(i) from  (
        select t.info i
           from test t
          where date >= initial_date
            limit 3
        )as s;

-- declare NOT FOUND handler
DECLARE CONTINUE HANDLER FOR NOT FOUND SET v_finished = 1;

set initial_date = '2013-01-01';

get_avg: LOOP

    OPEN cr;

    cr_loop: LOOP

        FETCH cr INTO v_avg;

        IF v_finished = 1 THEN 
            LEAVE cr_loop;
        END IF;

        insert into result values (v_avg);

    END LOOP cr_loop;

    CLOSE cr;

    #I've to change this line... the table does not have all days
    set v_dt_ini = date_add(v_dt_ini,interval 1 DAY);
    #But the problem it's not the line yet...
    set v_finished = 0;

    if v_dt_ini > '2013-05-30' then
        leave get_avg;
    end if;

END LOOP get_avg;

end$$

在我已将 N 固定为 3 的代码中 问题是，当我调用程序时......它计算 3 条记录的第一个平均值......并且当再次执行游标时......它返回相同的值......第 3 条记录......它不是我需要什么...

光标没有改变结果：/

Answer 1

SQLFiddle

SELECT date,info,(select avg(info) from
                  (select date,info,@row1:=@row1+1 as row
                   from test,(SELECT @row1:=0)r
                   order by date desc
                  )wrn1
                  where wrn1.row between wrn2.start and wrn2.end)as avg,
       start,end

FROM
    (select date,info,@row:=@row+1 as row,(@row - 2) as start,@row as end
     from test,(SELECT @row:=0)r
     order by date desc
    )wrn2
WHERE start > 0
ORDER BY date asc

这将返回

        DATE                   INFO     AVG       START     END
January, 01 2013 00:00:00+0000     210  212.3333    3        5
January, 02 2013 00:00:00+0000     213  208         2        4
January, 03 2013 00:00:00+0000     214  201         1        3

Answer 2

您可以为此使用相关子查询。如果您想要最多 n 天（包括当前日期和之后），那么这将起作用：

select date, info,
       (select avg(t2.info)
        from test t2
        where datediff(t.date, t2.date) < n and
              t2.date >= t.date
       ) as avginfo
from test t;

如果您希望 n 记录包括当前记录和之后的记录，则以下想法可行：

select date, info,
       (select avg(info)
        from (select t2.*
              from test t2
              where t2.date >= t.date
              order by t2.date
              limit n
             ) t3
       ) as avginfo
from test t;

但是，这会在 MySQL 中产生一个错误，我不确定如何解决它（双重嵌套查询无法再识别与外部查询的相关性）。以下版本确实如here所示工作：

select t.date, t.info, avg(t3.info)
from (select date, info,
             (select t2.date
              from test t2
              where t2.date >= t.date
              order by t2.date
              limit 1 offset 2
             ) as date3
      from test t
     ) t left outer join
     test t3
     on t3.date between t.date and t.date3
group by t.date, t.info;

Answer 3

回应关于被订购的评论 - 诀窍是排序：

新值：使用触发器 - 原始表上的AFTER INSERT 触发器需要选择最后 N 个值的平均值 (SELECT AVG(valuecolumn) FROM table WHERE datecolumn <= NEW.datecolumn ORDER BY datecolumn DESC LIMIT N) 并将其插入平均值表中。

现有值：从前到后运行此值，并使用 SELECT AVG(valuecolumn) FROM table WHERE datecolumn <= NEW.datecolumn ORDER BY datecolumn ASC LIMIT M,N，M 从 0..count-2 开始