Rechercher dans le manuel MySQL
12.25.2 DECIMAL Data Type Characteristics
This section discusses the characteristics of the
DECIMAL
data type (and its
synonyms), 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
The declaration syntax for a
DECIMAL
column is
DECIMAL(
.
The ranges of values for the arguments are as follows:
M
,D
)
M
is the maximum number of digits (the precision). It has a range of 1 to 65.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 thanM
.
If D
is omitted, the default is 0. If
M
is omitted, the default is 10.
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 differs from before.
Values for DECIMAL
columns are
stored using a binary format that packs nine decimal digits into 4
bytes. The storage requirements for the integer and fractional
parts of each value are determined separately. Each multiple of
nine digits requires 4 bytes, and any remaining digits left over
require some fraction of 4 bytes. The storage required for
remaining digits is given by the following table.
Leftover Digits | Number of Bytes |
---|---|
0 | 0 |
1–2 | 1 |
3–4 | 2 |
5–6 | 3 |
7–9 | 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 4 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 3 bytes for the remaining five
digits. The six fractional digits require 3 bytes.
DECIMAL
columns 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.
DECIMAL
columns do not permit
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 permits up to M
,D
)M
-
D
digits to the left of the decimal
point.
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 permits more. In MySQL,
DECIMAL(
and
M
,D
)NUMERIC(
are the same, and both have a precision of exactly
M
,D
)M
digits.
For a full explanation of the internal format of
DECIMAL
values, see the file
strings/decimal.c
in a MySQL source
distribution. The format is explained (with an example) in the
decimal2bin()
function.
Traduction non disponible
Le manuel MySQL n'est pas encore traduit en français sur l'infobrol. Seule la version anglaise est disponible pour l'instant.
Document créé le 26/06/2006, dernière modification le 26/10/2018
Source du document imprimé : https://www.gaudry.be/mysql-rf-precision-math-decimal-characteristics.html
L'infobrol est un site personnel dont le contenu n'engage que moi. Le texte est mis à disposition sous licence CreativeCommons(BY-NC-SA). Plus d'info sur les conditions d'utilisation et sur l'auteur.
Références
Ces références et liens indiquent des documents consultés lors de la rédaction de cette page, ou qui peuvent apporter un complément d'information, mais les auteurs de ces sources ne peuvent être tenus responsables du contenu de cette page.
L'auteur de ce site est seul responsable de la manière dont sont présentés ici les différents concepts, et des libertés qui sont prises avec les ouvrages de référence. N'oubliez pas que vous devez croiser les informations de sources multiples afin de diminuer les risques d'erreurs.