The Join stage does not provide reject handling for unmatched records (such as in an InnerJoin scenario). If un-matched rows must be captured or logged, an OUTER join operation must be performed. In an OUTER join scenario, all rows on an outer link (eg. Left Outer, Right Outer, or both links in the case of Full Outer) are output regardless of match on key values. 

During an Outer Join, when a match does not occur, the Join stage inserts NULL values into the unmatched columns. Care must be taken to change the column properties to allow NULL values before the Join. This is most easily done by inserting a Copy stage and mapping a column from NON-NULLABLE to NULLABLE.

 A Filter stage can be used to test for NULL values in unmatched columns.

 In some cases, it is simpler to use a Column Generator to add an ‘indicator’ column, with a constant value, to each of the outer links and test that column for the constant after you have performed the join. This is also handy with Lookups that have multiple reference links.