Groupby

Groups data by one or more attributes and/or aggregate measures.

Usage

groupby (indata, group-expr : outdata)

Details

Tuples of the relation indata are grouped by the attributes and aggregate functions specified in group-expr. The resulting outdata contains only those attributes and aggregate function results specified by group-expr.

Notes

Aggregate functions should follow the same format specified in the description of the Aggregate operator.

Known Bugs

Functions cannot accept literals.

Example

Using the input:

      RELATION books: title char, author char, pub_date date, pages number
      Title1|Author1|09-01-1991|34
      Title2|Author2|12-23-1954|479
      Title3|Author2|05-09-2002|733
      Title4|Author3|01-01-1968|32
      Title5|Author2|07-03-2001|1152

and the externally compiled MyFunctions.java that contains:

      import java.text.*;
      import java.util.*;
  
      public class MyFunctions
      {
        public static double average (ArrayList a_data)
        {
          return MyFunctions.sum(a_data)/((double)a_data.size());
        }
        public static double sum (ArrayList a_data)
        {
          double sum = 0;
          for (int i=0; i<a_data.size(); i++) {
            sum += Double.parseDouble(a_data.get(i).toString());
          }
          return sum;
        }
      }

when executing the plan:

      PLAN test
      {
        INPUT: stream books
        OUTPUT: stream result
  
        BODY
        {
          groupby (books, "author, MyFunctions.sum(pages) sum_pages, MyFunctions.average(pages) avg_pages" : result)
        }
      }

generates the following output:

      ----------------------------------------------
      RELATION: test
         attrs: author, sum_pages, avg_pages
      ----------------------------------------------
      Author2|2364.0|788.0
      Author3|32.0|32.0
      Author1|34.0|34.0
      ----------------------------------------------