MySQL 5.7 Reference Manual Including MySQL NDB Cluster 7.5 and NDB Cluster 7.6

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; ; that is,
`FLOAT(`

.
MySQL also supports this optional precision specification, but
the precision value in
* p*)

`FLOAT(``p`

)

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
`M`

`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 B.6.4.8, “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.