在SQL中计算的加权平均值

Question

I am trying to calculate the YTD value for every month. How could I achieve this in SQL Server?

Green is how my data looks in the table.I am suppose to calculate the one in Yellow. Your help will be much appreciated.

Year    Month   Monthly_Score   N_size  YTD_Score
2017    1        70             10       70*10/10
2017    2        80             20       70*10/(10+20) + 80 *20 /(10+20)
2017    3        90             30       70*10/(10+20+30) + 80 * 20 /(10+20+30) 
                                         +90*30/(10+20+30)

Answer 1

You can use the window function SUM if your version of SQL Server supports it.

select year,month,monthly_score,n_size,
1.0*sum(monthly_score*n_size) over(partition by year order by month)/
sum(n_size) over(partition by year order by month)
from yourtable

Note that the value resets every year.

Edit: For SQL Server versions below 2012 that support outer apply .

select y.year,y.month,y.monthly_score,y.n_size,t.ytd_score
from yourtable y
outer apply (select 1.0*sum(monthly_score*n_size)/sum(n_size) as ytd_score 
             from yourtable y1 
             where y.year=y1.year and y1.month<=y.month
            ) t

Answer 2

You can use the window variant of sum to sum to calculate running totals:

SELECT [Year], [Month], [Monthly_Score], [N_Size],
       SUM([Monthly_Score] * [N_Size]) 
          OVER (ORDER BY [Year], [Month]
          ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) / 
      SUM([N_Size]) 
          OVER (ORDER BY [Year], [Month]
          ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS YTD_Score
FROM  my_table