为什么SQL Server不返回0进行计算

Question

I have the following query:

select 
    (case 
        when try_convert(decimal(3, 1), DEC10assessmentData.writ_literatureReview) is not null 
           then cast(DEC10assessmentData.writ_literatureReview as decimal(3, 1)) 
           else 0 
     end / 100 * 4
     + case 
          when try_convert(decimal(3, 1), DEC10assessmentData.writ_exposition) is not null 
             then cast(DEC10assessmentData.writ_exposition as decimal(3, 1)) 
             else 0 
       end / 100 * 8
     + case 
          when try_convert(decimal(3, 1), DEC10assessmentData.writ_groupReport) is not null 
            then cast(DEC10assessmentData.writ_groupReport as decimal(3, 1)) 
            else 0 
       end / 100 * 8
     + case 
          when try_convert(decimal(3, 1), DEC10assessmentData.writ_synthSummary) is not null 
            then cast(DEC10assessmentData.writ_synthSummary as decimal(3, 1)) 
            else 0 
       end / 100 * 8
     + case 
          when try_convert(decimal(3, 1), DEC10assessmentData.writ_critEvaluation) is not null 
             then cast(DEC10assessmentData.writ_critEvaluation as decimal(3, 1))  
             else 0 
       end / 100 * 12) / 40 * 100
from 
    DEC10assessmentData

Why does it return 0.800000000 when all values in the selected columns are NULL ?

It returns correct results when columns have scores.

Answer 1

I tried creating demo here but I am getting 0.0000 , not 0.8000 . Are you sure that all values are null ?

Null Demo

If yes, the try running each case statement alone and see from which you are are not getting 0 in output.

So first run

select 
(case when try_convert(decimal(3,1), DEC10assessmentData.writ_literatureReview) 
is not null
   then 
cast(DEC10assessmentData.writ_literatureReview as decimal(3,1)
     ) else 0 
 end / 100 * 4
 )/ 40 * 100
from DEC10assessmentData

If this return only 0 then add another case statement and check further.