Another update from ingydotnet^djgoku

Task/Standard-deviation/SQL/standard-deviation.sql

@@ -0,0 +1,34 @@
+-- the minimal table
+create table if not exists teststd (n double precision not null);
+
+-- code modularity with view, we could have used a common table expression instead
+create view  vteststd as
+  select count(n) as cnt,
+  sum(n) as tsum,
+  sum(power(n,2)) as tsqr
+from teststd;
+
+-- you can of course put this code into every query
+create or replace function std_dev() returns double precision as $$
+ select sqrt(tsqr/cnt - (tsum/cnt)^2) from vteststd;
+$$ language sql;
+
+-- test data is: 2,4,4,4,5,5,7,9
+insert into teststd values (2);
+select std_dev() as std_deviation;
+insert into teststd values (4);
+select std_dev() as std_deviation;
+insert into teststd values (4);
+select std_dev() as std_deviation;
+insert into teststd values (4);
+select std_dev() as std_deviation;
+insert into teststd values (5);
+select std_dev() as std_deviation;
+insert into teststd values (5);
+select std_dev() as std_deviation;
+insert into teststd values (7);
+select std_dev() as std_deviation;
+insert into teststd values (9);
+select std_dev() as std_deviation;
+-- cleanup test data
+delete from teststd;