问题
Does anyone know about this?
回答1:
As the docs say:
M is the maximum number of digits (the precision). It has a range of 1 to 65. (Older versions of MySQL allowed a range of 1 to 254.)
D 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 M.
So M stands for Maximum (number of digits overall), D stands for Decimals (number of digits to the right of the decimal point).
回答2:
https://dev.mysql.com/doc/refman/5.7/en/precision-math-decimal-characteristics.html
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:
- M is the maximum number of digits (the precision). It has a range of 1 to 65. (Older versions of MySQL allowed a range of 1 to 254.)
- D 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 M.
[Note: the link above has been updated to point to the MySQL 5.7 docs, but the text was quoted from the MySQL 5.1 docs.]
回答3:
The doc says:
The declaration syntax for a DECIMAL column remains DECIMAL(M,D), although the range of values for the arguments has changed somewhat:
M is the maximum number of digits (the precision). It has a range of 1 to 65. This introduces a possible incompatibility for older applications, because previous versions of MySQL allow a range of 1 to 254. (The precision of 65 digits actually applies as of MySQL 5.0.6. From 5.0.3 to 5.0.5, the precision is 64 digits.)
D 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 M.
来源：https://stackoverflow.com/questions/1818046/what-does-m-d-mean-in-decimalm-d-exactly