uit de MySQL manual:
The NUMERIC and DECIMAL types are implemented as the same type by MySQL, as permitted by the SQL92 standard. They are used for values for which it is important to preserve exact precision,for example with monetary data. When declaring a column of one of these types the precision and scale can be (and usually is) specified;
The FLOAT type is used to represent approximate numeric data types. The ANSI/ISO SQL92 standard allows an optional specification of the precision (but not the range of the exponent) in bits following the keyword FLOAT in parentheses. The MySQL implementation also supports this optional precision specification. When the keyword FLOAT is used for a column type without a precision specification, MySQL uses four bytes to store the values. A variant syntax is also supported, with two numbers given in parentheses following the FLOAT keyword. With this option, the first number continues to represent the storage requirements for the value in bytes, and the second number specifies the number of digits to be stored and displayed following the decimal point (as with DECIMAL and NUMERIC). When MySQL is asked to store a number for such a column with more decimal digits following the decimal point than specified for the column, the value is rounded to eliminate the extra digits when the value is stored.