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What's wrong in this .. simple calculations

  • September 27, 2012 at 10:23 am

    #267391

    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

  • September 27, 2012 at 10:40 am

    #1543000

    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) -- <----<<<

  • September 27, 2012 at 10:55 am

    #1543009

    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

  • September 27, 2012 at 11:05 am

    #1543018

    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.:hehe:

  • September 27, 2012 at 11:13 am

    #1543021

    Discusses Issues with money.

    There are rounding issues with money and is probably not the best datatype to use in your scenario.

  • September 27, 2012 at 11:34 am

    #1543024

    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.

  • September 27, 2012 at 12:21 pm

    #1543050

    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.

    Backwards compatibility.

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  • March 6, 2013 at 3:42 am

    #1593847

    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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  • March 6, 2013 at 5:12 pm

    #1594222

    YOu may folow the money :hehe: 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.

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