CREATE SEQUENCE test_seq increment by 1 minvalue 1 nocycle noorder cache 20;
  
  SELECT case when rownum = 1 then 'x' else 'xxx' end test_data,
  case when rownum = 1 then test_seq.nextval else test_seq.currval end test_seq ,emp.* FROM emp;
  
  

SQL> create sequence s;

Sequence created.

SQL> create or replace function f (rn integer) return integer as 
  2    v pls_integer;
  3  begin
  4    if rn=1 then select s.nextval into v from dual;
  5    else select s.currval into v from dual;
  6    end if;
  7    return v;
  8  end;
  9  /

Function created.

SQL> select f(rownum), emp.ename
  2  from emp
  3  /
 F(ROWNUM) ENAME
---------- ----------
         1 SMITH
         1 ALLEN
         1 WARD
         1 JONES
         1 MARTIN
         1 BLAKE
         1 CLARK
         1 SCOTT
         1 KING
         1 TURNER
         1 ADAMS
         1 JAMES
         1 FORD
         1 MILLER

14 rows selected.