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MySQL 5.5 Reference Manual :: 12 Functions and Operators :: 12.18 Precision Math :: 12.18.2 DECIMAL Data Type Characteristics

- 12.18 Precision Math
- 12.18.1 Types of Numeric Values
- 12.18.2 DECIMAL Data Type Characteristics
- 12.18.3 Expression Handling
- 12.18.4 Rounding Behavior
- 12.18.5 Precision Math Examples

This section discusses the characteristics of the
`DECIMAL`

data type (and its
synonyms) in MySQL 5.5, 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 (prior to 5.0.3) are noted throughout this section.

The declaration syntax for a
`DECIMAL`

column is
`DECIMAL(`

.
The ranges of values for the arguments in MySQL 5.5
are as follows:
* M*,

`D`

is the maximum number of digits (the precision). It has a range of 1 to 65. (Older versions of MySQL permitted a range of 1 to 254.)`M`

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`D`

.`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 differs 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
5.5 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.

Unlike some older versions of MySQL,
`DECIMAL`

columns in MySQL
5.5 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
5.5 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 at most * M*,

`D`

`M`

`D`

`+`

sign.
The SQL standard requires that the precision of
`NUMERIC(`

be * M*,

`D`

`M`

`DECIMAL(``M`

,`D`

)

,
the standard requires a precision of at least
`M`

`DECIMAL(``M`

,`D`

)

and
`NUMERIC(``M`

,`D`

)

are the same, and both have a precision of exactly
`M`

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.

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*.

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