Special thanks to Ali and Adomas for providing the SP idea.
Below is working SP code, in case someone wanted to re-use it.
One thing I did not manage to work out is how to get the IB SQRT UDF
operational within the SP below.
I tried to declare the code below within the SP code but that did not
work.
DECLARE EXTERNAL FUNCTION SQRT
DOUBLE PRECISION
RETURNS DOUBLE PRECISION BY VALUE
ENTRY_POINT 'IB_UDF_sqrt' MODULE_NAME 'ib_udf';
At the end, I had to apply Delphi SQRT built-in function to the SP
output variable.

No doubt, someone will be able to provide a guidance on this last
obstacle.

Kind regards,
Mario

COMMIT WORK;
SET AUTODDL OFF;
SET TERM ^ ;

/* Stored procedures */

CREATE PROCEDURE "GETSTDDEV"
(
"FIRSTDATE" TIMESTAMP
)
RETURNS
(
"AVGINDEX" REAL,
"COUNTINDEX" INTEGER,
"STDDEV" REAL
)
AS
BEGIN EXIT; END ^


ALTER PROCEDURE "GETSTDDEV"
(
"FIRSTDATE" TIMESTAMP
)
RETURNS
(
"AVGINDEX" REAL,
"COUNTINDEX" INTEGER,
"STDDEV" REAL
)
AS
Begin
select avg(xindex), count(xindex) from xtable
where xDate > :FirstDate
into :avgindex,:countindex;
if (:countindex > 1) then
select (sum((xindex - :avgindex)*(xindex - :avgindex)) /
(:countindex - 1)) from xtable
where xDate > :FirstDate
into :stddev;
End
^

SET TERM ; ^
COMMIT WORK;
SET AUTODDL ON;

Note: the SP output variable stddev must be placed inside the external
sqrt function , e.g. in Delphi use: SQRT(stddev), in order to obtain
the actual Standard Deviation.

--- In firebird-support@yahoogroups.com, Ali Gökçen <alig@e...> wrote:
> Hi,
>
> if i remember correctly, STDDEV is:
>
> if(count(x)>1)
> sqrt ( sum((x-avg(x))*(x-avg(x))) / (count(x)-1) )
>
> Regards.
>
> Ali
>
> --- In firebird-support@yahoogroups.com, Adomas Urbanavicius
> <adomas@i...> wrote:
> > I wonder if it is not possible to write in one select:
> > (i dont remember exacty what is stddev, but if it is
> SQRT
> > (SUM( (x - mean(x) / N))
> > simple idea :
> > select
> > sqrt ( sum (x - avg(x))/count(x) )
> > from tb
> >
> > As nested agregations are not allowed,
> > in sql realization would be something like
> >
> > select
> > sqrt
> > (
> > sum
> > (x - (select avg(x) from tb)/
> > (select count(x) from tb)
> > )
> > )
> > from tb
> > Of course , this will be definetely slower than sample before in
> sp, so
> > final idea woul be :
> > create sp ....
> > select avg(x),count(x) from tb into avg_x,count_x
> > select sqrt(sum (x - avg_x)/count_x) from tb into res;
> >
> >
> > Ali G?n wrote:
> >
> > >Hey Mario,
> > >why are you so pessimist? what is to be a programmer?
> > > to be a drag-dropper? or visual screen graphic designer?
> > >
> > >if writing a STDDEV function using a procedure very difficult for
> > >you, then how may you write more complex programs?
> > >
> > >what is STDDEViation?
> > >
> > >n=total=0
> > >sampleavg = AVG(myfiled) -- use coalasce if there is null values
> > >
> > >for
> > >select samplevalue from mytable into :myvalue
> > >do
> > >begin
> > > n= n +1
> > > total = total + (myvalue - sampleavg)*(myvalue - sampleavg);
> > >end
> > >stddev = sqrrootfunctionofUDF(total / (n-1))
> > >
> > >if there is no sqrroot function in UDF, (i never need,
> > >so i dont now if exists) you can write one yourself using newton
> > >rapson method for example...
> > >
> > >Regards.
> > >
> > >Ali