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SQL Server decimal calculations showing odd results

Posted on 2012-03-22
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Last Modified: 2012-03-26
Consider the script below ran in Management Studio on SQLServer 2008:

          declare @N1 decimal(38,12)
          declare @N2 decimal(38,12)
          begin        
            set @N1=1111223333444455556666777.111222233
            set @N2=1111223333444455556666777.111222233
            select @N1/@N2  --expected result
            select @N1*cast(-1 as decimal(38,12))/@N2
            select @N1*(-1*(1.0))/@N2      
            select @N1*(-1.0/@N2)    
            select 1.0/@N2
          end;

The results are:
1.000000
-0.999999
-0.999999
-0.888979
0.0000000000000000000000008

Can anyone explain these results? Our problem is our software has to parse user friendly representation of field and number summing into sql friendly script which in turn gives correct and consistent results. We do need to deal with large number and many decimal places, hence using decimal(38,12). We are aware of the need to make the nominator of a division into a decimal number otherwise the result is assumed to be an integer,
ie
select 2/5 results in 0, and select 2.0/5 results in 0.4

We have also noticed that SQLServer doesn't always apply BODMAS calculation ordering. So, is there a set of rules that can be followed to achieve reliable results?

Many thanks
Alex
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Question by:stbdevelopment
5 Comments
 
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by:deighton
deighton earned 50 total points
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I think you are overflowing the maximum size of a float
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by:dqmq
dqmq earned 300 total points
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Interesting...seems like the more precision you give, the less you get back.  

         declare @N1 decimal(38,12)
            set @N1=1.111122223333
            select @N1
            select @N1/1
            select @N1/1.0
          select @N1/1.00
          select @N1/1.000
            select @N1/1.00000000000000000
            select @N1/cast(1 as decimal(38,12))
            select @N1/cast(1 as decimal(38,3))

-------------------

1.111122223333

1.111122223333

1.11112222333

1.1111222233

1.111122223

1.111122

1.111122

1.111122223
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Author Comment

by:stbdevelopment
Comment Utility
Thank you dqmq. That really highlights the problem we are having. However what is the solution?
Do we need to restrict the numbers used in calculations to so many decimal places and then cast them to a larger number of decimal places? But then the results would be the same. I can't see a way out of this.
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by:
dqmq earned 300 total points
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I don't know.  Still trying to make sense of it:

For more weirdness, try this:


           declare @N1 decimal(38,12)
            set @N1=   1.111122223333
            select @N1
          select @N1/1.0
          select @N1/1.000000000000
            select 1.111122223333/1.000000000000
            select 1.111122223333/1.0
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LVL 2

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by:winston33
winston33 earned 50 total points
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I'm sorry if this does not completely answer your question, but I generally complete the equation using the native datatypes (assuming the two numbers are using the same datatype) and then convert the result to the desired data type. The only statement below where this does not apply is the third because the "1" needs to be converted prior to the division. I cannot explain WHY this works, but I generally get the proper results. For example, using your example:

          declare @N1 decimal(38,12)
          declare @N2 decimal(38,12)
          begin        
            set @N1=1111223333444455556666777.111222233
            set @N2=1111223333444455556666777.111222233
            select CONVERT(DECIMAL(38,12), @N1/@N2)  --expected result            
            select CONVERT(DECIMAL(38,12), (@N1*-1)/@N2)            
            select CONVERT(DECIMAL(38,12), CONVERT(DECIMAL(38,12), 1)/@N2)
          end;


1.000000000000

-1.000000000000

0.000000000000 --> This is actually the correct result, I think, given the precision and scale.

Anyway, hope this helps a little.
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