Questions about 1Z0-047 Oracle Database SQL Expert

Question

I am planning to take this exam and I have several questions:
1) I am using Steve O'Hearn's 'SQL Certified Expert Exam Guide' book and this states the following about SQL functions:
"Be sure to review the Oracle Database SQL Language Reference Manual and review the lengthy description of all of the SQL functions before taking the exam"
In trial tests it seemed that book's information was enough, but how about real exam? Is it necessary to study something in addition to this book information? You can answer regarding other exam objectives as well, if there is something that I should read from some other materials.
2) The book states that I can add not null constraint to column that has null values, if I specify default value. I tried and cannot, I get error. So the book states it wrongly or do I misunderstand something?
3) The book states that I cannot drop a NOT NULL constraint, but I can get the job done using: ALTER TABLE table_name MODIFY column_name NULL;
I tried and I can execute: alter table table_name drop constraint nameofnotnullconstraint;
4) To use external tables, is only read grant on the directory necessary or also write?
5) I understood from the book that to flashback table (e.g to before drop) I need to have row movement enabled on the table. But I tried and I can make this flashback operation to table that does not have row movement enabled. How can this be explained!
Big thanks in advance!

Answer 1

#1) well, the manual is free; find here the SQL Language Reference - http://www.oracle.com/pls/db112/portal.all_books#index-SQL
#2 & 3) If you proved it yourself, that settles it!
#4) Here's a good article about external tables: http://www.oracle-developer.net/display.php?id=512
I noted this paragraph in it, which might answer your question:
In addition to the standard read-write Oracle directory that we need for our external table, we also need an additional executable directory object for the preprocessor. This directory defines the location of the executables used by the preprocessor (we will be using gzip below). As far as Oracle is concerned, an executable directory is one that has EXECUTE privileges granted on it (this is an 11g feature specifically to support the preprocessor).
#5) don't know