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Good afternoon.

Noticed the strangeness in calculating the amount in the field.

Vaues table:

Vaues table

Type field - float.

I make select sum:

SELECT SUM(cost) as cost FROM Table

In result i get sum = 20.47497010231018;

sum result

I use calculator) and i get sum = 20,47497

Tell me please why do I get different results?

Leo Loki
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  • yeah, basically that's a floating point common issue. do not use float as a type in mysql, you have other data types that will do the trick. – Sebas Feb 19 '13 at 21:34
  • @Sebas tell me more please, what types of fields can replace the float? – Leo Loki Feb 19 '13 at 21:35
  • Time to read "What Every Computer Scientist Should Know About Floating Point Arithmetic" http://download.oracle.com/docs/cd/E19957-01/806-3568/ncg_goldberg.html – Sean McSomething Feb 19 '13 at 21:36
  • Those two numbers are almost identical (assuming that your calculator is using European format and your MySql is in American format). Why do you call them "very different"? Or is it just the formatting issue? – RBarryYoung Feb 19 '13 at 21:37
  • @Loki, it depends what is the business meaning of what you're storing in that column. (money...) – Sebas Feb 19 '13 at 21:38
  • @RBarryYoung i dont said **very** different. i sai ifferent because the results are not the same and I need to round the value received in the request... – Leo Loki Feb 19 '13 at 21:45
  • @LeoLoki Your title does say "**very** different". But that's OK, I was making sure that that was what you meant. – RBarryYoung Feb 19 '13 at 21:46
  • @Sebas yes, this column stores the value of money. – Leo Loki Feb 19 '13 at 21:54
  • @RBarryYoung sorry - i see now that title have **very**. i clear this) thanks. – Leo Loki Feb 19 '13 at 21:55

3 Answers3

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Here, read this: http://floating-point-gui.de/

So, the problem is that floating point numbers - the internal implementation for decimal numbers in hardware, and used by most languages and applications, does not map 1 to 1 to numbers in base 10, as we are used. The underlying values are expressed in base 2, and have a limited precision. Some numbers that can be represented with few digits in the decimal notation can actually need much more digits in the native format - leading to rounding errors.

The article linked above explains it in detail.

jsbueno
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These kinds of differences are not enough to be worried about in most situations.

Floating point numbers are notorious for being slightly off of the intended values because of the number of bits and how floating point numbers are stored in binary. For example, as demonstrated here, a decimal value of 0.1 has an actual double value of 0.10000000149011612. Close, but not exact.

A technique I've seen used in some applications that need absolutely accurate latitude and longitude numbers is that they'll keep the values in integral data types that are equivalent to the float multiplied by some power of 10. For example, a GeoPoint in the Google Maps API v1 on Android measures in micro degrees. Instead of a latitude and longitude like 54.123428, 60.809234 to preserve the values precisely they'd be ints: 54123428, 60809234. To depict this, they'd call the variables latitudeE6 and longitudeE6 to indicate that it's the real latitude or longitude multiplied by 1E6 (the scientific notation of 10^6).

Corey Ogburn
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Use DECIMAL(16, 4) (for example) type for your currency columns, where 4 is the number of digits after the .

You will have to trim the trailing zeros though, since for instance 10 would appear 10.0000. This is however easily done.

Sebas
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