如何从SQL Server中获取上个月的数据

Question

I have 2 tables ms and fr .

ms contains columns pid,month , and totalsales , while fr contains pid, month , and forecast .

In both tables, each pid has 12 record based on month.

I actually want to calculate the demand where the formula is (0.3 * totalsales) + (0.7 * previous month result of forecast) . The previous month result of forecast is like when the user choose pid 2 and month 2, then it will take the forecast result from month 1 as its calculation data.

I tried it already but the desired result is not returned:

select 
    ((0.3 * totalsalesamount) + (0.7 * forecastdemand)) as demand 
from 
    Monthlysales, forecastreorder 
where 
    Monthlysales.P_ID = forecastreorder.Productid 
    and Monthlysales.P_ID = 1 
    and forecastreorder.Month = 2

When I execute the code above the result is based on their each forecast result. For example, when I choose pid 1, month 2, then it will take the forecast result from month 2 also. meanwhile i want it to take the forecast result from month 1 as its calculation data.

Answer 1

尝试此操作以获取上个月的SQL

SELECT Convert(datetime, DateAdd(month, -1, Convert(date, GetDate())));

Answer 2

Using a similar table but with a year field, using ISNULL for no found data from previous moth, presume the previous is the month sales.

SELECT    
   0.3 * ms.totalsalesamount + 0.7 * ISNULL(fr.forecastdemand, ms.totalsalesamount) as demand
FROM            
   ms 
LEFT OUTER JOIN
   fr ON ms.pid = fr.pid 
      AND fr.month = CASE WHEN ms.month > 1 THEN ms.month - 1 ELSE 12 END 
      AND fr.year = CASE WHEN ms.month > 1 THEN ms.year ELSE ms.year - 1 END