Pentaho Kettle scripting option

Question

I am trying to use the Pentaho Kettle software for a few transformations on my largetables. I want to perform an operation that displays the contents of alternate rows in two different tables and then I wish to join the two tables later for further transformation.

The scripting option in the tool helps me with the executing SQL scripts for single row or multiple rows. Can anyone help me with how to select the row for this purpose.