Question

Is there an elegant way to delete all non-selected entries from a table after selecting from it?

Answer 1

I think you mean DELETE and not DROP. DROP can only erase an entire object (e.g., TABLE). If you aren’t worried about performance, you can have something like

DELETE FROM mytable WHERE mytable_key NOT IN 
  (SELECT mytable_key FROM mytable WHERE some_or_another_condition);

Many DBs allow a JOIN-type syntax that will probably perform better if you have to do this on a frequent basis.