### http://dev.mysql.com/doc/refman/5.5/en/floating-point-types.html 에서 참조

### 11.2.2. Fixed-Point Types (Exact Value) – `DECIMAL`

, `NUMERIC`

The `DECIMAL`

and `NUMERIC`

types store exact numeric data values. These types are used when it is important to preserve exact precision, for example with monetary data. In MySQL, `NUMERIC`

is implemented as `DECIMAL`

, so the following remarks about `DECIMAL`

apply equally to `NUMERIC`

.

MySQL 5.5 stores `DECIMAL`

values in binary format. See Section 12.18, “Precision Math”.

In a `DECIMAL`

column declaration, the precision and scale can be (and usually is) specified; for example:

salary DECIMAL(5,2)

In this example, `5`

is the precision and `2`

is the scale. The precision represents the number of significant digits that are stored for values, and the scale represents the number of digits that can be stored following the decimal point.

Standard SQL requires that `DECIMAL(5,2)`

be able to store any value with five digits and two decimals, so values that can be stored in the `salary`

column range from `-999.99`

to `999.99`

.

In standard SQL, the syntax `DECIMAL(`

is equivalent to * M*)

`DECIMAL(``M`

,0)

. Similarly, the syntax `DECIMAL`

is equivalent to `DECIMAL(``M`

,0)

, where the implementation is permitted to decide the value of *. MySQL supports both of these variant forms of*

`M`

`DECIMAL`

syntax. The default value of *is 10.*

`M`

If the scale is 0, `DECIMAL`

values contain no decimal point or fractional part.

The maximum number of digits for `DECIMAL`

is 65, but the actual range for a given `DECIMAL`

column can be constrained by the precision or scale for a given column. When such a column is assigned a value with more digits following the decimal point than are permitted 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 permissible number of digits.)

The `FLOAT`

and `DOUBLE`

types represent approximate numeric data values. MySQL uses four bytes for single-precision values and eight bytes for double-precision values.

For `FLOAT`

, the SQL standard permits an optional specification of the precision (but not the range of the exponent) in bits following the keyword `FLOAT`

in parentheses. MySQL also supports this optional precision specification, but the precision value is used only to determine storage size. A precision from 0 to 23 results in a 4-byte single-precision `FLOAT`

column. A precision from 24 to 53 results in an 8-byte double-precision `DOUBLE`

column.

MySQL permits a nonstandard syntax: `FLOAT(`

or * M*,

*)*

`D`

`REAL(``M`

,`D`

)

or `DOUBLE PRECISION(``M`

,`D`

)

. Here, “`(``M`

,`D`

)

” means than values can be stored with up to *digits in total, of which*

`M`

*digits may be after the decimal point. For example, a column defined as*

`D`

`FLOAT(7,4)`

will look like `-999.9999`

when displayed. MySQL performs rounding when storing values, so if you insert `999.00009`

into a `FLOAT(7,4)`

column, the approximate result is `999.0001`

.Because floating-point values are approximate and not stored as exact values, attempts to treat them as exact in comparisons may lead to problems. They are also subject to platform or implementation dependencies. For more information, see Section C.5.5.8, “Problems with Floating-Point Values”

For maximum portability, code requiring storage of approximate numeric data values should use `FLOAT`

or `DOUBLE PRECISION`

with no specification of precision or number of digits.