MySQL 8.0 Reference Manual Including MySQL NDB Cluster 8.0

14.24.2 DECIMAL Data Type Characteristics

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

The declaration syntax for a DECIMAL column is DECIMAL(M,D). The ranges of values for the arguments are as follows:

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. (There is also a limit on how long the text of DECIMAL literals can be; see Section 14.24.3, “Expression Handling”.)

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(M,D) column permits up to M - D digits to the left of the decimal point.

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 permits more. In MySQL, DECIMAL(M,D) and NUMERIC(M,D) are the same, and both have a precision of exactly 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.