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21.2. DECIMAL Data Type Changes

This section discusses the characteristics of the DECIMAL data type (and its synonyms) as of MySQL 5.0.3, with particular regard to the following topics:

Some of these changes result in possible incompatibilities for applications that are written for older versions of MySQL. These incompatibilities are noted throughout this section.

The declaration syntax for a DECIMAL column remains DECIMAL(M,D), although the range of values for the arguments has changed somewhat:

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. (Prior to MySQL 5.0.3, decimal values could have up to 254 digits. However, calculations were done using floating-point and thus were approximate, not exact.) This change in the range of literal values is another possible source of incompatibility for older applications.

Values for DECIMAL columns no longer are represented as strings that require one byte per digit or sign character. Instead, a binary format is used that packs nine decimal digits into four bytes. This change to DECIMAL storage format changes the storage requirements as well. 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 digits left over require some fraction of four bytes. 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(20,10) column has ten digits on either side of the decimal point. Each part requires four bytes for nine of the digits, and one byte for the remaining digit.

The storage required for leftover digits is given by the following table:

Leftover DigitsNumber of Bytes
00
11
21
32
42
53
63
74
84
94

As a result of the change from string to numeric format for DECIMAL storage, DECIMAL columns no longer store a leading + character or leading 0 digits. Before MySQL 5.0.3, if you inserted +0003.1 into a DECIMAL(5,1) column, it was stored as +0003.1. As of MySQL 5.0.3, it is stored as 3.1. Applications that rely on the older behavior must be modified to account for this change.

The change of storage format also means that DECIMAL columns no longer support the non-standard extension that allowed values larger than the range implied by the column definition. Formerly, one byte was allocated for storing the sign character. For positive values that needed no sign byte, MySQL allowed an extra digit to be stored instead. For example, a DECIMAL(3,0) column must support a range of at least –999 to 999, but MySQL would allow storing values from 1000 to 9999 as well, by using the sign byte to store an extra digit. This extension to the upper range of DECIMAL columns no longer is allowed. In MySQL 5.0.3 and up, a DECIMAL(M,D) column allows at most MD digits to the left of the decimal point. This can result in an incompatibility if an application has a reliance on MySQL allowing “too-large” values.

The SQL standard requires that the precision of NUMERIC(M,D) be exactly M digits. For DECIMAL(M,D), the standard requires a precision of at least M digits but allows more. In MySQL, DECIMAL(M,D) and NUMERIC(M,D) are the same, and both have a precision of exactly M digits.

Summary of incompatibilities:

The following list summarizes the incompatibilities that result from changes to DECIMAL column and value handling. You can use it as guide when porting older applications for use with MySQL 5.0.3 and up.

The behavior used by the server for DECIMAL columns in a table depends on the version of MySQL used to create the table. If your server is from MySQL 5.0.3 or higher, but you have DECIMAL columns in tables that were created before 5.0.3, the old behavior still applies to those columns. To convert the tables to the newer DECIMAL format, dump them with mysqldump and reload them.

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