Column Alias Logic!

Question

Hi,
I'm in the process of learning Oracle9i SQL and I don't
understand why column alias are not allowed in a where
and group by clause but are allowed in the order by
clause? To me this makes no sense and I have noticed that
that there are a lot of conditions in SQL that make no
sense (ie. this is allowed in a from clause but not the where clause unless using this function or that). Too
many conditions and restrictions are in place!

Answer 1

Exactly. Database vendors make their own enhancements and additions, but if you don't like the core SQL language you should take it up with ANSI.
That FLAGGER switch is neat:
SQL> set flagger off
SQL> SELECT DECODE('x','y','z','Using default') AS example1 FROM dual;
EXAMPLE1
Using default
SQL> SELECT CASE 'x' WHEN 'y' THEN 'z' ELSE 'Using default' END AS example2 FROM dual;
EXAMPLE2
Using default
SQL> set flagger full
SQL> SELECT DECODE('x','y','z','Using default') AS example1 FROM dual;
SELECT DECODE('x','y','z','Using default') AS example1 FROM dual
ERROR at line 1:
ORA-00097: use of Oracle SQL feature not in SQL92 Full Level
ORA-06550: line 2, column 8:
PLS-01416: Use of <id> (<value>...) here
SQL> SELECT CASE 'x' WHEN 'y' THEN 'z' ELSE 'Using default' END AS example2 FROM dual;
EXAMPLE2
Using default