I have found some of ways to implement this project using C++, SQL, java, or C.

As I had mentioned earlier, I was searching for some of database systems that have this facility or in some way can implement it.

The database system that I discovered while learning for this topic is HP Vertica and in this system the basic approach is creating 

user defined library saved as .so for C/C++ or .jarr for java and loading it using CREATE LIBRARY command. When we call SQL function HP Vertica passes data values to the code in the library to process it.

CREATE [ OR REPLACE ] AGGREGATE FUNCTION [[db-name.]schema.]function-name
... AS LANGUAGE 'language' NAME 'factory' LIBRARY library_name;

example using this method is as given below

 => CREATE LIBRARY AggregateFunctions AS '/opt/vertica/sdk/examples/build/AggregateFunctions.so';

CREATE LIBRARY
=> CREATE AGGREGATE FUNCTION ag_avg AS LANGUAGE 'C++' NAME 'AverageFactory'
   library AggregateFunctions;
CREATE AGGREGATE FUNCTION
=> CREATE AGGREGATE FUNCTION ag_cat AS LANGUAGE 'C++' NAME 'ConcatenateFactory'
   library AggregateFunctions;
CREATE AGGREGATE FUNCTION
=> \x
Expanded display is on.
select * from user_functions;

The only drawback in this method that this can be implemented using C++ only for HP Vertica.

The another method that I discovered is given in the latest update of oracle i.e Oracle 9i (9)

the refrence link for same is http://www.oracle-developer.net/display.php?id=215

According to this we require two components to implement aggregate functions

2. a PL/SQL function.

There is a development framework for data cartridges (think of this as a template) provided by Oracle. This provides the structure of the object type and its methods (down to the detail of how we name them) and also defines how we create our PL/SQL function. For example, our object type will contain the following:attribute(s) for holding state information. These can be of any existing built-in or user-defined datatype;

• a mandatory ODCIAggregateInitialize static method to reset the state attributes at the start of an aggregation;
  
• a mandatory ODCIAggregateIterate member method to apply each input value to the running aggregate value;
  
• a mandatory ODCIAggregateTerminate member method to return the final result of the aggregate; and
  
• an optional ODCIAggregateMerge member method, used to combine the results of more than one stream of aggregation (for example, with parallel query) before returning a result.
  

Please let me know whether I am working in right direction or not and whether the methods I have mentioned can be used or not to implement the project.

Regards,

Soham Mankad