decimal 关键字表示128 位数据类型。同浮点型相比,decimal 类型具有更高的精度和更小的范围,这使它适合于财务和货币计算。
From decimal.c(mysql):

  /**
      intg is the number of *decimal* digits (NOT number of decimal_digit_t's !)
           before the point
      frac is the number of decimal digits after the point
      len  is the length of buf (length of allocated space) in decimal_digit_t's,
           not in bytes
      sign false means positive, true means negative
      buf  is an array of decimal_digit_t's
   */
  typedef struct st_decimal_t {
    int    intg, frac, len;
    my_bool sign;
    decimal_digit_t buf[9];
  } decimal_t;

c# decimal 参考:
http://msdn.microsoft.com/zh-cn/library/364x0z75%28v=vs.90%29.aspx
http://msdn.microsoft.com/zh-cn/library/system.decimal%28v=vs.110%29.aspx