Baffling SQL failure in Oracle - anyone explain?

In response to a post on comp.databases I did a bit of testing and found a very strange result.

This works in Firebird (and something similar apparently works in MySQL)

SELECT DISTINCT(First_Name)
FROM Employee
ORDER BY Last_Name

There is a sample db Employee, somewhat similar to Scott.Emp.

However these both fail in Oracle

SELECT DISTINCT(Ename)
FROM Emp
ORDER BY Sal

or even ORDER BY Emp.Sal

with the error

ERROR at line 3:
ORA-01791: not a SELECTed expression

Now, I can't for the life of me figure out why this query is failing in Oracle - I think that it's perfectly reasonable to sort on a column that one isn't selecting on.

This is so simple that I'm asking myself if I've overlooked something really *_really_* obvious.

Paul...

-- 

plinehan __at__ yahoo __dot__ __com__

XP Pro, SP 2, 

Oracle, 9.2.0.1.0 (Enterprise Ed.)
Interbase 6.0.1.0;

When asking database related questions, please give other posters 
some clues, like operating system, version of db being used and DDL.
The exact text and/or number of error messages is useful (!= "it didn't work!").
Thanks.
 
Furthermore, as a courtesy to those who spend 
time analysing and attempting to help, please 
do not top post.