The DOUBLE function returns a floating-point number corresponding
to a:

- number if the argument is a numeric expression.
- character string representation of a number if the argument is a string expression.

DOUBLE [PRECISION] (NumericExpression)

- NumericExpression
- The argument is an expression that returns a value of any built-in numeric data type.

The result of the function is a double-precision floating-point number. If the argument can be null, the result can be null; if the argument is null, the result is the null value. The result is the same number that would occur if the argument were assigned to a double-precision floating-point column or variable.

DOUBLE (StringExpression)

- StringExpression
- The argument can be of type CHAR or VARCHAR in the form of a numeric constant. Leading and trailing blanks in argument are ignored.

The result of the function is a double-precision floating-point number. The result can be null; if the argument is null, the result is the null value. The result is the same number that would occur if the string was considered a constant and assigned to a double-precision floating-point column or variable.