Quote
... implicit join ...
To
what, exactly?
Your example has only a simple, Parent-Child relationship.
Most databases have
far more than one child table, so which table(s) would you implicitly join to? You certainly don't want to implicitly join across
every child table, because
(a) that's going to be hideously inefficient if you don't actually need any data from those other tables and
(b) MySQL only allows for 61(?) tables in a "join" anyway. More than that and "Boom!".
I don't see why this doesn't do what you want:
SELECT directory_name
FROM table1 t1
LEFT JOIN table2 t2
ON t1.data_field = t2.key_field
WHERE t2.key_field IS NULL
Regards, Phill W.