This section discusses the characteristics of the DECIMAL data type (and its synonyms) in MySQL 5.1, with particular regard to the following topics:

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(M,D). The ranges of values for the arguments in MySQL 5.1 are as follows:

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 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:

Unlike some older versions of MySQL (prior to 5.0.3), DECIMAL columns in MySQL 5.1 do not store a leading + character or leading 0 digits. If you insert +0003.1 into a DECIMAL(5,1) column, it is stored as 3.1. 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(M,D) column allows at most 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(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.

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.