Selecting an average of records grouped by 5 minut

问题:

I'm having a slight issue. I have a PostgreSQL table with such format

time (datetime)     | players (int) | servers (int)
---------------------------------------------------
2013-12-06 13:40:01 | 80            | 20
2013-12-06 13:41:13 | 78            | 21
etc.

I would like to group them by 5 minute periods and get an average of the group as a single value, so there will be 20% of the records, each containing an average of ~5 numbers, with time set to the first time value in the group. I have no idea how to do this in PgSQL. So the result would be:

2013-12-06 13:40:01 | avg of players on :40, :41, :42, :43, :44 | same with servers
2013-12-06 13:45:05 | avg of players on :45, :46, :47, :48, :49 | same with servers
2013-12-06 13:50:09 | avg of players on :50, :51, :52, :53, :54 | same with servers
2013-12-06 13:55:12 | avg of players on :55, :56, :57, :58, :59 | same with servers

回答1:

SELECT grid.t5
      ,min(t."time") AS min_time
--    ,array_agg(extract(min FROM t."time")) AS 'players_on' -- optional
      ,avg(t.players) AS avg_players
      ,avg(t.servers) AS avg_servers
FROM (
   SELECT generate_series(min("time")
                         ,max("time"), interval '5 min') AS t5
   FROM tbl
   ) grid
LEFT JOIN tbl t ON t."time" >= grid.t5
               AND t."time" <  grid.t5 +  interval '5 min'
GROUP  BY grid.t5
ORDER  BY grid.t5;

Explain

• The subquery grid produces one row for every 5 minutes from the minimum to the maximum of "time" in your table.

• LEFT JOIN back to the table slicing data in 5-min intervals. Carefully include lower border and exclude upper border.

• To drop 5-min-slots where nothing happened, use JOIN in place of LEFT JOIN.

• To have your grid-times start at 0:00, 5:00 etc, round down the min("time") in generate_series().

More explanation in these related answers:
Group by data intervals
PostgreSQL: running count of rows for a query 'by minute'

Aside: I wouldn't use time as identifier. It's a reserved word in standard SQL and a function / type name in Postgres.

回答2:

Try this, it should group minutes 0-4, 5-9, 10-14 and so on...

SELECT MIN(time), AVG(Players), AVG(Servers)
FROM MyTable t
GROUP BY date_trunc('hour', time),
    FLOOR(datepart('minute', time)/12)

EDIT: Changed the grouping to hour first and then to the Floor of minutes. I Think this should work.

回答3:

How about this?

select datepart('year', time) as StartYear, datepart('month', time) as StartMonth,
    datepart('day', time) as StartDay, datepart('hour', time) as StartHour,
    floor(datepart('minute', time)/5)*5 as StartMinute,
    avg(case when datepart('minute', time) = floor(datepart('minute', time)/5)*5 then players else null end) as Zero,
    avg(case when datepart('minute', time) = floor(datepart('minute', time)/5)*5+1 then players else null end) as One,
    avg(case when datepart('minute', time) = floor(datepart('minute', time)/5)*5+2 then players else null end) as Two,
    avg(case when datepart('minute', time) = floor(datepart('minute', time)/5)*5+3 then players else null end) as Three,
    avg(case when datepart('minute', time) = floor(datepart('minute', time)/5)*5+4 then players else null end) as Four,
from MyTable
group by datepart('year', time), datepart('month', time),
    datepart('day', time), datepart('hour', time),
    floor(datepart('minute', time)/5)*5