message: Add description for test data.
diff: @@ -10,6 +10,23 @@ using namespace std;
  #include <iostream>
  +/*
  +test data
  +create table foo (i int, j int);
  +
  +insert into foo values
  +(10, 1),
  +(10, 1),
  +(20, 2),
  +(20, 2),
  +(40, 4),
  +(40, 4),
  +(20, 1),
  +(20, 1),
  +(10, 2),
  +(10, 2);
  +*/
  +
  /* boss case
  SELECT i, SUM(i), SUM(i), COUNT(*), AVG(i), j
  FROM foo
  