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What's wrong in this .. simple calculations Expand / Collapse
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Posted Thursday, September 27, 2012 10:23 AM
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If I run the following on SQL Server 2012 I get two different answers from the first two select statments.

Any reason why and what I need to do to avoid this. The second statement gives the wrong answer and that is the statement that I need to use in my code. With the change in statement 3 , I get the right answer but I am not getting the confidence that I should introduce the change in all my calculations.

select round((1400.05 / 4232.33 ) * 4180.06 ,2) --Statement 1

declare @v1 smallmoney = 1400.05,
@v2 smallmoney = 4232.33,
@v3 smallmoney = 4180.06

select round((@v1 / @v2 ) * @v3 ,2) --Statement 2

select round((@v1*1.0 / @v2 ) * @v3 ,2) --Statement 3


Thank you very much for the help.

Regards
Divesh
Post #1365374
Posted Thursday, September 27, 2012 10:40 AM


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I think the intermediate result from @v1 / @v2 is being truncated because you're using smallmoney data type.

Can you use a different datatype, or cast one value to decimal to make it clearer what's happening.

e.g.

select round((CAST(@v1 as decimal(18,2)) / @v2 ) * @v3 ,2) --Statement 2

or


declare @v1 smallmoney = 1400.05,
@v2 smallmoney = 4232.33,
@v3 smallmoney = 4180.06

select round((CAST(@v1 as decimal(18,2)) / @v2 ) * @v3 ,2), --Statement 2
CAST(@v1 as decimal(18,2)) / @v2,
CAST(round((CAST(@v1 as decimal(18,2)) / @v2 ) * @v3 ,2) as SmallMoney) -- <----<<<


Post #1365385
Posted Thursday, September 27, 2012 10:55 AM
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Laurie,

Thank you for your reply.

I guess if I force a cast to a decimal it will work since that is what happens when I add the "* 1.0" but I thought money or smallmoney was a decimal number.

Even if I change the datatype to money, I get the same results.

If we have to force a cast or conversion to a decimal type then does it mean that there is no point in using the money or smallmoney data type? !!! I hope this is not the conclusion of this.

Thanks for any more leads on this.

Regards
Divesh
Post #1365395
Posted Thursday, September 27, 2012 11:05 AM


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Money & Smallmoney both have 4 decimal places, so that's why they lose the detail. If you're looking for accuracy it's better to use decimal.

It's all a bit of a minefield.

Post #1365405
Posted Thursday, September 27, 2012 11:13 AM
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Discusses Issues with money.
There are rounding issues with money and is probably not the best datatype to use in your scenario.

[url=http://www.sqlservercentral.com/Forums/Topic544518-9-1.aspx][/url]
Post #1365408
Posted Thursday, September 27, 2012 11:34 AM
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diveshps (9/27/2012)
Laurie,

Thank you for your reply.

I guess if I force a cast to a decimal it will work since that is what happens when I add the "* 1.0" but I thought money or smallmoney was a decimal number.

Even if I change the datatype to money, I get the same results.

If we have to force a cast or conversion to a decimal type then does it mean that there is no point in using the money or smallmoney data type? !!! I hope this is not the conclusion of this.

Thanks for any more leads on this.

Regards
Divesh


Speaking for myself, I always use DECIMAL to store dollar amounts to avoid exactly this sort of issue. I've never seen the point of money or smallmoney.
Post #1365412
Posted Thursday, September 27, 2012 12:21 PM


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cphite (9/27/2012)

Speaking for myself, I always use DECIMAL to store dollar amounts to avoid exactly this sort of issue. I've never seen the point of money or smallmoney.


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Post #1365444
Posted Wednesday, March 6, 2013 3:42 AM


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This behaviour is due to data type precedence.

(@v1 / @v2 ) * @v3 will return smallmoney
((@v1 * 1.0) / @v2 ) * @v3 will return numeric

This can be demonstrated like this:

DECLARE @v1 SMALLMONEY = 1400.05,
@v2 SMALLMONEY = 4232.33;

SELECT @v1 / @v2 AS Val INTO TestTable1
SELECT @v1 / @v2 * 1.0 AS Val INTO TestTable2

EXEC sp_help TestTable1
EXEC sp_help TestTable2

DROP TABLE TestTable1
DROP TABLE TestTable2


And you will see the following:

TestTable1

Name - Val
Type - smallmoney
Length - 4
Precision - 10
Scale - 4

TestTable 2

Name - Val
Type - numeric
Length - 9
Precision - 13
Scale - 5




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Post #1427272
Posted Wednesday, March 6, 2013 5:12 PM
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YOu may folow the money by tracing intermediate results:

select (1400.05 / 4232.33 ), (1400.05 / 4232.33 ) * 4180.06, round((1400.05 / 4232.33 ) * 4180.06 ,2) --Statement 1

declare @v1 smallmoney = 1400.05,
@v2 smallmoney = 4232.33,
@v3 smallmoney = 4180.06

select @v1, (@v1 / @v2 ), (@v1 / @v2 ) * @v3, round((@v1 / @v2 ) * @v3 ,2) --Statement 2

select @v1*1.0, (@v1*1.0 / @v2 ) ,(@v1*1.0 / @v2 ) * @v3 , round((@v1*1.0 / @v2 ) * @v3 ,2) --Statement 3

For most accurate calculations always use FLOAT data type (unless you need to handle more than 15 digigts, than use "big" decimals).
Try use FLOAT in your example and you'll see that 4 digit returned by DECIMAL calculation is incorrect.
Fortunateluy for you, it's beyond rounding precision.

Post #1427698
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