Sauirabh, IsNumeric() returns 1 if the parameter could be converted to a number - but it doesn't have to be a numeric. So IsNumerc('123') will return 1, because you can easily convert string '123' to a number 123. More information can be found in MSDN article on IsNumeric().

As for the length constraint - according to the MS SQL Data Types list, "Floating precision number data with the following valid values: -1.79E + 308 through -2.23E - 308, 0 and 2.23E + 308 through 1.79E + 308". Such numbers have 309 digits - hence the limitation.

However, either I don't understand something or there's something wrong, because my MS SQL 2005 installation returns the following:

-- Returns 0, 1 respectively

SELECT IsNumeric('2' + REPLICATE('0', 308)), IsNumeric('179' + REPLICATE('0', 306));

So numbers between 1.79E + 308 and 2.23E + 308 seem not to be taken into account.