PLS SPOT THE ERROR

Question

SELECT NAME FROM EX WHERE NAME IS NULL UNION ALL
SELECT NAME FROM EX WHERE NAME IS NOT NULL ORDER BY DECODE(NAME,'MASTER',0,1) NULLS LAST,NAME
what is the error in the above query
i am getting the foll error
SQL Error: ORA-01785: ORDER BY item must be the number of a SELECT-list expression
01785. 00000 - "ORDER BY item must be the number of a SELECT-list expression"
*Cause:   
*Action:                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       

Answer 1

Although your query is pointless you need to reference the columns in your order by correctly. Unions cause issues with referring to the columns by name so the easiest way it to wrap the query in another select...
SQL> ed
Wrote file afiedt.buf
  1  select ename from (
  2  select ename
  3  from emp
  4  where ename is null
  5  union all
  6  select ename
  7  from emp
  8  where ename is not null
  9  )
10* order by decode(ename,'SCOTT',0,1) NULLS LAST, ename
SQL> /
ENAME
SCOTT
ADAMS
ALLEN
BLAKE
CLARK
FORD
JAMES
JONES
KING
MARTIN
MILLER
SMITH
TURNER
WARD
WILLIS
15 rows selected.
SQL>