A tip to keep in mind is to not think of a join in SQL terms. If you have table A and you join table B to it, the result is one table still called 'table A' with all the rows of table B appended and B's additional columns added.
KEEP acts similar to a join, but a) the result is still having two separate tables, and b) data could be reduced (ie. removed from) one or the other or both tables depending on if you do LEFT, RIGHT, or INNER KEEP.
