This reminded me of one more difference between Windows and Linux/Unix.
MySQL use the round function out of the host libraries. If you are on a
Windows box the rule for rounding is if the column immediately to the right
of the column you are rounding to is a 5 then round up
i.e. make 2.485 >>> 2.49
make 2.595 >>> 2.60
If you are on a Linux/Unix box the rule for rounding is if the column
immediately to the right of the column you are rounding to is a 5 then round
up if the column you are rounding to is odd and round down if the column you
are rounding to is even
i.e. make 2.485 >>> 2.49
make 2.595 >>> 2.59
Windows Linux/Unix
2.4850 2.49 2.48
2.5950 2.60 2.60
2.7700 2.77 2.77
7.8500 7.86 7.85
This was run on a RedHat server
mysql> select round(2.4850,2), round(2.5950,2), round(2.7700,2),
round(2.4850,2)+round(2.5950,2)+round(2.7700,2);
+-----------------+-----------------+-----------------+---------------------
----------------------------+
| round(2.4850,2) | round(2.5950,2) | round(2.7700,2) |
round(2.4850,2)+round(2.5950,2)+round(2.7700,2) |
+-----------------+-----------------+-----------------+---------------------
----------------------------+
| 2.48 | 2.60 | 2.77 |
7.85 |
+-----------------+-----------------+-----------------+---------------------
----------------------------+
1 row in set (0.00 sec)
-----Original Message-----
From: Martin [mailto:martin.y@stripped]
Sent: Monday, February 21, 2005 7:49 PM
To: Hassan Schroeder; mysql@stripped
Subject: Re: Odd rounding errors with 4.1
Huh, you know. Now that I'm not at work and therefore don't have my
numbers to check against, you're right.
Man, I must need more coffee.
Never mind me. :)
May be back tomorrow, though, when I have the numbers in front of me. I
know they didn't add up earlier...
Martin
Hassan Schroeder wrote:
> Martin wrote:
>
>> My recent test involved the following three values from the column:
>> 2.4950
>> 2.5950
>> 2.7700
>
>
>> When I use a SUM() on these I get: 7.860
>
>
> Sounds good to me...
>
>> If I switch the column over to a FLOAT, then the SUM() becomes
>> 7.8599998950958
>>
>> Using Excel to test the numbers, or hand-calculating, I get:
>> 7.8550.
>
>
> Time for a hand upgrade, I think :-) 5 + 5 = 5??? I don't even
> want to think about how Excel would come up with this...
>
>> Shouldn't the SUM() remain with the precision of the DECIMAL type and
>> not try to round to 2 decimal places?
>
>
> My own, possibly suspect, hand calculations show that SUM() is right;
> and it's common knowledge that floating point isn't the right thing to
> use for situations like this -- that's why there *is* a DECIMAL type.
>
> FWIW!
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