SQL> with t as (
  2  select s.nextval,deptno from (select distinct deptno from emp) t)
  3  select * from t
  4  /
select s.nextval,deptno from (select distinct deptno from emp) t)
         *
ERROR at line 2:
ORA-02287: sequence number not allowed here


SQL> create or replace function stax_sq return number is begin return s.nextval;end;
  2  /

Function created.


SQL> ed
Wrote file afiedt.buf

  1  with t as (
  2  select stax_sq(),deptno from (select distinct deptno from emp) t)
  3* select * from t
SQL> /

 STAX_SQ()     DEPTNO
---------- ----------
       282         30
       283         20
       284         10

SQL>

SQL> set pages 50000
SQL> l
  1  with t as (
  2             select distinct stax_sq() seq,deptno from (select distinct deptno from emp) t
  3             )
  4  select  t.seq,
  5          e.deptno,
  6          e.ename
  7    from  emp e,
  8          t
  9*   where e.deptno = t.deptno
SQL> /

       SEQ     DEPTNO ENAME
---------- ---------- ----------
       305         30 ALLEN
       305         30 WARD
       305         30 MARTIN
       305         30 BLAKE
       305         30 TURNER
       305         30 JAMES
       307         10 CLARK
       307         10 KING
       307         10 MILLER
       306         20 SMITH
       306         20 JONES
       306         20 SCOTT
       306         20 ADAMS
       306         20 FORD

14 rows selected.

SQL>