Join stage: Mapping tab
For the Join stage, on the Output tab select Edit in the Columns section. Here, you can specify how the output columns are derived, that is, what input columns map onto them.
The left pane shows the input columns from the links whose tables have been joined. These are read only and cannot be modified on this tab.
The right pane shows the output columns for the output link. This has a Derivations field where you can specify how the column is derived. You can fill it in by dragging input columns over, or by using the Auto-match facility.