本节讨论 DECIMAL 数据类型（及其同义词）的特性，特别关注以下主题：This section discusses the characteristics of the DECIMAL data type (and its synonyms), with particular regard to the following topics:

DECIMAL 列的声明语法为 DECIMAL(M,D)。参数的值范围如下：The declaration syntax for a DECIMAL column is DECIMAL(M,D). The ranges of values for the arguments are as follows:

如果省略D，默认值为 0。如果省略M，默认值为 10。If D is omitted, the default is 0. If M is omitted, the default is 10.

M的最大值为 65，意味着在DECIMAL值上的计算精确到 65 位数字。这个 65 位精度的限制也适用于精确值的数字字面量，因此此类字面量的最大范围与之前不同。（DECIMAL字面量的文本长度也有限制；参见第 14.24.3 节，“表达式处理”。）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”.)

对于 DECIMAL 列的值使用二进制格式存储，每 4 字节打包九个十进制数字。每个值的整数部分和小数部分的存储需求分别确定。每九位数字需要 4 字节，剩余的数字则需要 4 字节的一部分。剩余数字所需的存储空间如下表所示。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.

例如，一个 DECIMAL(18,9) 列在小数点的两侧各有九位数字，因此整数部分和小数部分各需要 4 个字节。一个 DECIMAL(20,6) 列有十四位整数数字和六位小数数字。整数数字中九位需要 4 个字节，其余五位需要 3 个字节。六位小数数字需要 3 个字节。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 列不会存储前导的 + 字符、- 字符或前导 0 数字。如果你将 +0003.1 插入到 DECIMAL(5,1) 列中，它会被存储为 3.1。对于负数，字面上的 - 字符不会被存储。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 列不允许超出列定义所暗示的范围的值。例如，DECIMAL(3,0) 列支持的范围是 -999 到 999。DECIMAL(M,D) 列允许小数点左边最多有 M - D 位数字。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.

SQL 标准要求 NUMERIC(M,D) 的精度 恰好 为 M 位数字。对于 DECIMAL(M,D)，标准要求精度至少为 M 位数字，但允许更多。在 MySQL 中，DECIMAL(M,D) 和 NUMERIC(M,D) 是相同的，并且两者的精度都恰好为 M 位数字。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.

有关 DECIMAL 值内部格式的完整说明，请参阅 MySQL 源代码分发中的文件 strings/decimal.c。该格式在 decimal2bin() 函数中有解释（附例子）。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.