Predicting and Controlling Join Output

In the previous activity your join generated one row: the result of a combination of the incoming row in the stream and the one row in the window that met the join condition.

Let's see what happens if we send a row into TradesInquiries that has no corresponding trade record in TradesWindow.

  1. Restart project.
  2. Go back to the insert row box and enter the value YHOO in the box and click send.
  3. Look at the viewers again. Notice that TradeResults does not have any output for this. Since the ON clause didn't find a match in the window for the incoming YHOO symbol, nothing was published.
  4. Stop the project.

Exercise

Change the KEEP clause in the Create Window to KEEP 3 ROWS PER Symbol. If you manually send the same value into the stream as you did in the previous exercise, how many lines of output might it generate in the results stream? Restart the project and try sending the row into the stream. What rows in the window was it joined with? Stop the project and change the KEEP clause back to KEEP LAST ROW PER Symbol before proceeding.

Parent topic: Joins