EXISTS and IN operator

Question

SELECT emp_name
FROM emp E
WHERE EXISTS ( SELECT 'X'
FROM dept
WHERE dept_no = E.dept_no
AND dept_cat = 'A');
SELECT emp_name
FROM emp E
WHERE dept_no IN ( SELECT dept_no
FROM dept
WHERE dept_no = E.dept_no
AND dept_cat = 'A');
SELECT emp_name
FROM dept D, emp E
WHERE E.dept_no = D.dept_no
AND D.dept_cat = 'A';
The article says that none of these are consistently faster; it depends on your data . I am confused regarding the statement . Can anyone please explain those circumstances (data) !
Responses are appreciated!

Answer 1

and also read:
Oracle® Database Performance Tuning Guide
10g Release 2 (10.2)
Part Number B14211-01
Paragraph titled as:
11.5.3.4 Use of EXISTS versus IN for Subqueries
In general , Oracle proposes to use:
IN
if the selective predicate is in the subquery
EXISTS
f the selective predicate is in the parent query
Look at examples in the paragraph reffered above....
Greetings..
Sim