Trouble with MySQL query using AVG()

问题

I am using a query that takes an average of all the records for each given id...

$query = "SELECT bline_id, AVG(flow) as flowavg 
          FROM blf 
          WHERE bline_id BETWEEN 1 AND 30 
          GROUP BY bline_id 
          ORDER BY bline_id ASC";

These records are each updated once daily. I would like to use only the 10 most recent records for each id in my average.

Any help would be qreatly appreciated.

blf table structure is:

id | bline_id | flow | date

回答1:

Another option is to simulate ROW_NUMBER().

This statement creates a counter and resets it every time it encounters a new bline_id. It then filters out any records that aren't in the first 10 rows.

SELECT bline_id, 
       Avg(flow) avg 
FROM   (SELECT id, 
               bline_id, 
               flow, 
               date, 
               CASE 
                 WHEN @previous IS NULL 
                       OR @previous = bline_id THEN @rownum := @rownum + 1 
                 ELSE @rownum := 1 
               end rn, 
               @previous := bline_id 
        FROM   blf, 
               (SELECT @rownum := 0, 
                       @previous := NULL) t 
        WHERE bline_id > 0 and bline_id < 31
        ORDER  BY bline_id, 
                  date DESC, 
                  id) t 
WHERE  rn < 11
GROUP  BY bline_id

DEMO

It's worthwhile seeing this in action by removing the group by and looking at intermediate results

回答2:

If these are really updated every day, then use date arithmetic:

SELECT bline_id, AVG(flow) as flowavg
FROM blf
WHERE bline_id BETWEEN 1 AND 30 and
      date >= date_sub(now(), interval 10 day)
GROUP BY bline_id
ORDER BY bline_id ASC

Otherwise, you have to put in a counter, which you can do with a correlated subquery:

SELECT bline_id, AVG(flow) as flowavg
FROM (select blf.*,
             (select COUNT(*) from blf blf2 where blf2.bline_id = blf.bline_id and blf2.date >= blf.date
             ) seqnum
      from blf
     ) blf
WHERE bline_id BETWEEN 1 AND 30 and
      seqnum <= 10
GROUP BY bline_id
ORDER BY bline_id ASC

来源：https://stackoverflow.com/questions/15622791/trouble-with-mysql-query-using-avg

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