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From:Jerry Schwartz Date:November 8 2010 8:57pm
Subject:RE: numeric comparisons
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The two different versions of MySQL might perform the calculations in a 
slightly different order.

By the way, it isn't just comparing numbers that can cause misleading results. 
Consider the following.


10000000000 + 1.5 + 7 - 10000000000

What is the result of that calculation?

It depends upon the native precision of the machine, how the language chooses 
to cast the operands, and how (or if) the compiler chooses to reorder the 
operations.

I would not rely on the result being the same if you changed software 
versions, let alone languages.


Regards,

Jerry Schwartz
Global Information Incorporated
195 Farmington Ave.
Farmington, CT 06032

860.674.8796 / FAX: 860.674.8341
E-mail: jerry@stripped
Web site: www.the-infoshop.com

>-----Original Message-----
>From: Larry Martell [mailto:larry.martell@stripped]
>Sent: Monday, November 08, 2010 3:12 PM
>To: Michael Satterwhite
>Cc: mysql@stripped
>Subject: Re: numeric comparisons
>
>On Mon, Nov 8, 2010 at 1:01 PM, Michael Satterwhite <michael@stripped>
>wrote:
>> While I don't know why the behavior changed, the comparison of floating 
>> point
>> values has been an issue since the day computers were created. In the last 
>> 10
>> - 15 years, the comparisons have worked better, but it's still an issue. 
>> The
>> problem is that most decimal fractions do not convert well to binary.
>> Actually, I'd expect the example of 4.5 to work fine as 0.5 does convert 
>> well
>> to a binary floating point - but that is academic.
>>
>> In the 1970's I got used to writing floating point comparisons as
>>
>>        x = 4.3
>>        if(abs(x - 4.3) .lt. <some tolerance>)
>>
>> The tolerance is how close the values needed to be for me to consider them
>> equal. An example might be
>>
>>        if(abs(x - 4.3) .lt. 0.00001)
>>
>> Someone else might be able to shed some light on why the behavior changed.
>>
>> On Monday, November 08, 2010 12:50:16 pm Larry Martell wrote:
>>> I have a client that asked me to look into a situation where they were
>>> seeing different behavior with the same data and same sql on 2
>>> different servers.
>>>
>>> The have some sql that was comparing a double to a varchar in a where
>>> clause - something like this:
>>>
>>> where (doubleCol > varcharCol and some other conditions) or
>>>            (doubleCol < varcharCol and some other conditions)
>>>
>>> Let's take an example where the varcharCol has a string in like
>>> '4.5000' and the doubleCol has 4.5.
>>>
>>> On their 5.0.45 server this was working as 'expected' (i.e. the
>>> comparisons in the where clause were false since they are numerically
>>> equal). But on their 5.0.18 server the > clause was resolving to true.
>>>  I changed the query to cast both columns to binary, so the query now
>>> reads:
>>>
>>> where (cast(doubleCol as binary) > cast(varcharCol as binary) and some
>>> other conditions) or
>>>            (cast(doubleCol as binary) < cast(varcharCol as binary) and
>>> some other conditions)
>>>
>>> And now on both servers the < clause of the query is resolving to true.
>>>
>>> I know I can strip off the trailing zeros from the varchar, but there
>>> must be a generic way to cast these so they compare properly as
>>> numbers.
>>>
>>> TIA
>>> -larry
>
>
>Prompted by a reply I got off-list, I changed the cast from casting to
>binary to casting to decimal(10,5) and it seems to work properly on
>both servers.
>
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Thread
numeric comparisonsLarry Martell8 Nov
  • Re: numeric comparisonsMichael Satterwhite8 Nov
    • Re: numeric comparisonsLarry Martell8 Nov
      • RE: numeric comparisonsJerry Schwartz8 Nov
  • Re: numeric comparisonsPeter Brawley8 Nov