11.2.2. Fixed-Point Types (Exact Value) –
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
MySQL 5.5 stores
DECIMAL values in binary format. See Section 12.18, “Precision Math”.
DECIMAL column declaration, the precision and scale can be (and usually is) specified; for example:
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
In standard SQL, the syntax
DECIMAL( is equivalent to
DECIMAL(. Similarly, the syntax
DECIMAL is equivalent to
DECIMAL(, where the implementation is permitted to decide the value of
M. MySQL supports both of these variant forms of
DECIMAL syntax. The default value of
M is 10.
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.)
DOUBLE types represent approximate numeric data values. MySQL uses four bytes for single-precision values and eight bytes for double-precision values.
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
MySQL permits a nonstandard syntax:
DOUBLE PRECISION(. Here, “
(” means than values can be stored with up to
M digits in total, of which
D digits may be after the decimal point. For example, a column defined as
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
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
DOUBLE PRECISION with no specification of precision or number of digits.