[Q] Why cant I ORDER BY on a SELECT...UNION in PL/SQL

Why wont this work in pl/sql (but will otherwise):

select col1,col2 from tab1
union
select col1,col2 from tab2
order by col1;

Get message PLS-00423: ORDER BY item must be the number of a SELECT-list expression

Oracle support in the UK dont appear to know why it wont work. Everything works until you add the order by clause. Received on Mon Feb 23 1998 - 00:00:00 CST