why if you do this conversion you change the result?

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why if you do

SQL

declare @p float='29545428.022495'

select cast(@p as numeric(17,9))

you obtain this?
29545428.022495002

please can you explain the differece of this type and help me to understand why the value change?
thank you

Posted 25-Aug-15 11:01am

gregorio89

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PIEBALDconsult 25-Aug-15 17:07pm

It doesn't. Avoid floats.

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F-ES Sitecore · Accepted Answer · 2015-08-25T12:23:00

Solution 1

You'll always get slight differences converting between float\numeric types in sql (double\decimal in c#) because the float\double type doesn't hold the exact number you want, it holds a mathematical representation that is close to what you want. This allows it to store far greater\smaller numbers than their "actual number" equivalents like numeric\decimal. If numeric precision is crucial to you then floats are not a good choice.

Posted 25-Aug-15 12:23pm

F-ES Sitecore

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gregorio89 26-Aug-15 3:22am

but why exists a data type that can't store the perfect assigned value ?

F-ES Sitecore 26-Aug-15 3:52am

Numbers take up space, and a system can only do maths etc on numbers up to a certain size. If you want really really big, or really really small numbers then you are going beyond the standard number size limits so the language allows the use of fuzzy variables that can hold bigger values as the values themselves are not held literally, but a mathematical approximation of them is.

It's your job as a developer to fully understand the underlying data types and language features and use what is appropriate.

gregorio89 26-Aug-15 5:03am

Thank you