ORDER BY with DISTINCT gives ORA-01791: not a SELECTed expression - sql

Question

I trying to use oracle order by with select statement but it causes an Exception:
ORA-01791: not a SELECTed expression.
select distinct usermenu.menuname
from usermenu, userpermission
where userpermission.menuno = usermenu.menuno
and userpermission.userno = 1
order by userpermission.menuno;

Answer 1

When there's DISTINCT or an aggregate function in the SELECT statement's column list, ORDER BY a column which isn't part of the SELECT column list won't work.
Here's an example, based on Scott's schema.
This works OK, although D.LOC isn't selected:
SQL> select d.dname, e.ename
2 from dept d join emp e on e.deptno = d.deptno
3 order by d.loc;
DNAME ENAME
-------------- ----------
SALES BLAKE
SALES TURNER
SALES ALLEN
SALES MARTIN
SALES WARD
SALES JAMES
RESEARCH SCOTT
RESEARCH JONES
RESEARCH SMITH
RESEARCH ADAMS
RESEARCH FORD
ACCOUNTING KING
ACCOUNTING MILLER
ACCOUNTING CLARK
14 rows selected.
Now, add DISTINCT - basically, that's what you have:
SQL> select distinct d.dname, e.ename
2 from dept d join emp e on e.deptno = d.deptno
3 order by d.loc;
order by d.loc
*
ERROR at line 3:
ORA-01791: not a SELECTed expression
The same goes for aggregate functions, such as COUNT:
SQL> select d.dname, e.ename, count(*)
2 from dept d join emp e on e.deptno = d.deptno
3 group by d.dname, e.ename
4 order by d.loc;
order by d.loc
*
ERROR at line 4:
ORA-00979: not a GROUP BY expression
SQL>
So, what to do? Order by something else. Alternatively, use the current query as an inline view, join it with the table that contains the column you'd want to order the result by and it'll work:
SQL> select x.dname, x.ename
2 from (select distinct d.dname, e.ename
3 from dept d join emp e on e.deptno = d.deptno
4 ) x
5 join dept d1 on d1.dname = x.dname
6 order by d1.loc;
DNAME ENAME
-------------- ----------
SALES TURNER
SALES JAMES
SALES BLAKE
SALES WARD
SALES MARTIN
SALES ALLEN
RESEARCH SMITH
RESEARCH FORD
RESEARCH ADAMS
RESEARCH SCOTT
RESEARCH JONES
ACCOUNTING MILLER
ACCOUNTING KING
ACCOUNTING CLARK
14 rows selected.
SQL>