This section discusses the characteristics of the
DECIMAL
data type (and its
synonyms) in MySQL 5.1, with particular regard to the
following topics:
Maximum number of digits
Storage format
Storage requirements
The nonstandard MySQL extension to the upper range of
DECIMAL
columns
Possible incompatibilities with applications that are written for older versions of MySQL are noted throughout this section.
The declaration syntax for a
DECIMAL
column is
DECIMAL(
.
The ranges of values for the arguments in MySQL 5.1
are as follows:
M
,D
)
M
is the maximum number of digits
(the precision). It has a range of 1 to 65. (Older versions of
MySQL allowed a range of 1 to 254.)
D
is the number of digits to the
right of the decimal point (the scale). It has a range of 0 to
30 and must be no larger than M
.
The maximum value of 65 for M
means
that calculations on DECIMAL
values
are accurate up to 65 digits. This limit of 65 digits of precision
also applies to exact-value numeric literals, so the maximum range
of such literals is different from before. (In older versions of
MySQL, decimal values could have up to 254 digits. However,
calculations were done using floating-point and thus were
approximate, not exact.)
Values for DECIMAL
columns in MySQL
5.1 are stored using a binary format that packs nine
decimal digits into four bytes. The storage requirements for the
integer and fractional parts of each value are determined
separately. Each multiple of nine digits requires four bytes, and
any remaining digits left over require some fraction of four
bytes. The storage required for remaining digits is given by the
following table.
Leftover Digits | Number of Bytes |
0 | 0 |
1 | 1 |
2 | 1 |
3 | 2 |
4 | 2 |
5 | 3 |
6 | 3 |
7 | 4 |
8 | 4 |
9 | 4 |
For example, a DECIMAL(18,9)
column has nine
digits on either side of the decimal point, so the integer part
and the fractional part each require four bytes. A
DECIMAL(20,6)
column has fourteen integer
digits and six fractional digits. The integer digits require four
bytes for nine of the digits and three bytes for the remaining
five digits. The six fractional digits require three bytes.
Unlike some older versions of MySQL (prior to 5.0.3),
DECIMAL
columns in MySQL
5.1 do not store a leading +
character or -
character or leading
0
digits. If you insert
+0003.1
into a DECIMAL(5,1)
column, it is stored as 3.1
. For negative
numbers, a literal -
character is not stored.
Applications that rely on the older behavior must be modified to
account for this change.
DECIMAL
columns in MySQL
5.1 do not allow values larger than the range implied
by the column definition. For example, a
DECIMAL(3,0)
column supports a range of
-999
to 999
. A
DECIMAL(
column allows at most M
,D
)M
-
D
digits to the left of the decimal
point. This is not compatible with applications relying on older
versions of MySQL that allowed storing an extra digit in lieu of a
+
sign.
The SQL standard requires that the precision of
NUMERIC(
be exactly M
,D
)M
digits. For
DECIMAL(
,
the standard requires a precision of at least
M
,D
)M
digits but allows more. In MySQL,
DECIMAL(
and
M
,D
)NUMERIC(
are the same, and both have a precision of exactly
M
,D
)M
digits.
For more detailed information about porting applications that rely
on the old treatment of the DECIMAL
data type, see the MySQL 5.0 Reference
Manual.
User Comments
There are programming languages and database accessing interfaces (e.g. Delphi 7 using DBExpress) which handle MySQL database columns with a NUMERIC type as BCD values, and have their own limitation of assuming that the fractional part has an even number of digits to use bytes optimally.
Although MySQL will be able to store odd numbers of fractional digits, such columns may not be handled correctly by those languages and interfaces.
Example: I used a column of the type NUMERIC(6,3) and got values in return which were off by the factor 16 in an application written in Delphi 7, using the DBExpress interface.
As far as I know, recent PHP 4 and 5 versions don't have issues with such columns.
Add your own comment.