Float type in MySQL

Question

I have a MySQL table with column of type float(10, 6).

If I insert 30.064742 into the column the value stored in the database is 30.064741.

Why?

Answer 1

Floating-point numbers imply a certain amount of imprecision. Use a DECIMAL column if you need to be certain to retain every digit.

Answer 2

It's a general problem with rounding numbers to a precision which can be stored in the database. Floats will round to multiples of powers of two. If you want something that is easier to think about, you can use the Decimal type, which will round to powers of ten.

More details in the documentation for numeric types:

When such a column is assigned a value with more digits following the decimal point than are allowed by the specified scale, the value is converted to that scale. (The precise behavior is operating system-specific, but generally the effect is truncation to the allowable number of digits.)