Say I own a costume shop or something a long those lines.
tableA holds the information for the costume.
If you want to see what items in TableA that are not rented out you can restate the question as which items doesn't have a rented out period that the current date falls in
Expressed in SQL it would be a a query using a negated
exists predicate with a correlated subquery, like this:
select * from TableA a where not exists ( select * from TableB b where b.barcode = a.barcode and now() between hiredOn and dueBack )
There are many other ways to achieve the same result, but I think this fits the semantics of the question well.