> Well, let us assume a table T had an int field N, the table has 4 rows:
> {10,12,14,16}
>
> I need to do this:
>
> SELECT std_udf(N) FROM T;
>
> And it will return one value:
>
> 2.236
>
> Thus I need to 'create a custom aggregate function' or simple a
> "Aggregate UDF"
>

not in one call. You need to process the calculation in a stored procedure
at this time, using the available UDFs and built-in functions
Alan