如何获得Microsoft SQL Server 2008中计数的平均值

Question

How can I get the average of this count?

SELECT
    driver,
    COUNT(order_number) as month_total
FROM 
    Unloads
WHERE 
    completiondate BETWEEN '11-06-2017' AND '11-10-2017'
GROUP BY 
    driver
ORDER BY 
    month_total DESC;

Answer 1

This may seem like overkill, but data over a time series can have gaps, so to reliably average by month you need to ensure there are no gaps. A way (there are many) is to do something like the following so that the ranges exist as rows, then left join the data to those. The other thing I particularly caution you on is to avoid using "between" for date ranges. Use >= and < as you see below this ensures you don't accidentally over count for data sitting one the upper boundary dates.

declare @start_dt date;
set @start_dt = '20170601';

select
       u.driver, period_start_dt, period_end_dt
     , count(completiondate)            montly_count
     , AVG(COUNT(completiondate))OVER() avg_montly_count
from (
        select 
               dateadd(month,m.n,start_dt)   period_start_dt
             , dateadd(month,m.n+1,start_dt) period_end_dt
             , m.n                           month_no
        from (
               select @start_dt start_dt ) seed
        cross join (select 0 n union all
                    select 1 union all select 2 union all select 3 union all
                    select 4 union all select 5 union all select 6 union all
                    select 7 union all select 8 union all select 9 union all
                    select 10 union all select 11
                   ) m
     ) r

-- LEFT JOIN YOUR DATA
LEFT JOIN unloads u on u.completiondate >= r.period_start_dt and u.completiondate < r.period_end_dt

WHERE period_start_dt >= '20170601' and period_start_dt < '20171001'

group by
       u.driver , period_start_dt, period_end_dt      
order by 
       period_start_dt,  u.driver

Demo

Answer 2

If you want the average of the driver by day- you can subquery and divide the count for each driver by the number of days passed. Number of days passed is the datediff of the two dates plus 1 day. It is plus 1 day since you have used BETWEEN, which is inclusive. IOW, for days 6, 7, 8, 9, and 10- this is 5 days. But datediff(DAY, '2017-11-06', '2017-11-10') is only 4 days- so you must add +1 day to divide by the correct number of days. So the algorithm looks something like:

SELECT dT.driver
      ,month_total / (DATEDIFF(DAY, '2017-11-06', '2017-11-10') + 1) [Daily Driver Avg]

  FROM (
        SELECT
            driver,
            COUNT(order_number) as month_total
        FROM 
            Unloads
        WHERE 
            completiondate BETWEEN '2017-11-06' AND '2017-11-10'
        GROUP BY 
            driver
       ) AS dT