http://www.runoob.com/mysql/mysql-data-types.html
decimal
DECIMAL column is DECIMAL(M,D)
The ranges of values for the arguments are as follows:
M is the maximum number of digits (the precision). It has a range of 1 to 65.
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.
PS：Each multiple of nine digits requires 4 bytes
https://dev.mysql.com/doc/refman/5.7/en/precision-math-decimal-characteristics.html
JPA
24位有效数字，其中10位有效小数
1 | (precision = 24, scale = 10, nullable = false) |
最佳实践
在《Effective Java》这本书中也提到这个原则，float和double只能用来做科学计算或者是工程计算，在商业计算中我们要用java.math.BigDecimal。
使用BigDecimal并且一定要用String来够造。
精度
double
8 bytes
有符号
(1.797 693 134 862 315 7 E+308，2.225 073 858 507 201 4 E-308)，0，(2.225 073 858 507 201 4 E-308，1.797 693 134 862 315 7 E+308)
无符号
0，(2.225 073 858 507 201 4 E-308，1.797 693 134 862 315 7 E+308)