How to append a ResultSet to another ResultSet?

Question

Hi,
How could I append a ResultSet to another ResultSet object if their columns are exactly the same?
For example, I have some methods that return ResultSet objects for particular queries.
int a = 3;
ResultSet rs1 = getRoute(20, a);
ResultSet rs2 = getRoute(a, 40);
ResultSet rs3 = rs1 + rs2;
How do I combine the two ResultSet objects together instead of writing the last statement? Thank you.
-Henry Chang

Answer 1

As I noted earlier, UNION is always going to re-sort your results and remove any duplicate rows; UNION ALL might or might not depending on the database implementation.
I think Oracle happens to be among the databases that will produce a concatenated result FROM UNIUN ALL, without resorting, but if not, and if your database happens to be Oracle, then this will work:
SELECT * FROM
  (SELECT test_table.*, 1 subresult_no, rownum row_no
  FROM test_table
  WHERE column_a = 20 AND column_b = 10
  ORDER BY order_no)
UNION ALL
  (SELECT *, 2 subresult_no, rownum row_no
  FROM test_table
  WHERE column_a = 10 AND column_b = 30
  ORDER BY order_no DESC)
ORDER BY subresult_no, rownum row_no"rownum" is an Oracle feature, it's a pseudo-column that gives the row index at that point in the computation.
The above SQL will of course add 2 columns to your result set; you can of course suppress them by explicitly listing your selected columns; doing so is widely considered to be good practice anyway.