Columns to match against each other.

The join transformation takes values in a particular column of the primary table (on the left here) and finds all rows that match the given column of the secondary table (on the right here) and shows them as a single row in the result.You can select multiple columns here, in which case only the rows where all pairs of columns match will be included. If you select no columns, all possible pairings of the primary and secondary rows will be included.