SQL> DECLARE
  2  s VARCHAR2(200);
  3  n number;
  4  
  5  BEGIN
  6  
  7  FOR  i in (SELECT order_number FROM oe_order_headers_all WHERE rownum<10) LOOP
  8  DBMS_OUTPUT.PUT_LINE(i.order_number);
  9  END LOOP;
 10  
 11  
 12  END;
 13  /
1
1
2
2
3
3
4
5
6

PL/SQL procedure successfully completed.

SQL> DECLARE
  2  BEGIN
  3  
  4  FOR  i in (SELECT order_number||'.'||(SELECT level
  5            FROM DUAL
  6            CONNECT BY level=1) 
  7      FROM oe_order_headers_all WHERE rownum<10) LOOP
  8  DBMS_OUTPUT.PUT_LINE(i.order_number);
  9  END LOOP;
 10  END;
 11  
 12  /
DECLARE
*
ERROR at line 1:
ORA-06550: line 4, column 39:
PLS-00103: Encountered the symbol "SELECT" when expecting one of the following:
( - + case mod new not null others <an identifier>
<a double-quoted delimited-identifier> <a bind variable> avg
count current exists max min prior sql stddev sum variance
execute forall merge time timestamp interval date
<a string literal with character set specification>
<a number> <a single-quoted SQL string> pipe

scott@ORA92> DECLARE
  2  BEGIN
  3    FOR i IN (SELECT deptno
  4                     || '.'
  5                     || (SELECT  level
  6                         FROM    DUAL
  7                         CONNECT BY level = 1)
  8                       AS some_alias
  9              FROM   dept
 10              WHERE  ROWNUM < 10)
 11    LOOP
 12      DBMS_OUTPUT.PUT_LINE (i.some_alias);
 13    END LOOP;
 14  END;
 15  /
10.1
20.1
30.1
40.1
50.1

PL/SQL procedure successfully completed.

SQL> DECLARE
  2  s VARCHAR2(200);
  3  n number;
  4  
  5  BEGIN
  6  
  7  FOR  i in (SELECT order_number||'.'||(SELECT level FROM DUAL  CONNECT BY level=1) as a
  8      FROM oe_order_headers_all WHERE rownum<10) LOOP
  9  DBMS_OUTPUT.PUT_LINE(i.a);
 10  END LOOP;
 11  
 12  END;
 13  /
DECLARE
*
ERROR at line 1:
ORA-06550: line 7, column 39:
PLS-00103: Encountered the symbol "SELECT" when expecting one of the following:
( - + case mod new not null others <an identifier>
<a double-quoted delimited-identifier> <a bind variable> avg
count current exists max min prior sql stddev sum variance
execute forall merge time timestamp interval date
<a string literal with character set specification>
<a number> <a single-quoted SQL string> pipe


SQL> 

scott@ORA92> DECLARE
  2  BEGIN
  3    FOR i IN (SELECT deptno
  4              FROM   dept)
  5    LOOP
  6      FOR j IN
  7        (SELECT empno
  8         FROM   emp
  9         WHERE  deptno = i.deptno)
 10      LOOP
 11        DBMS_OUTPUT.PUT_LINE (i.deptno || '.' || j.empno);
 12      END LOOP;
 13    END LOOP;
 14  END;
 15  /
10.7782
10.7839
10.7934
20.7369
20.7566
20.7788
20.7876
20.7902
30.7499
30.7521
30.7654
30.7698
30.7844
30.7900

PL/SQL procedure successfully completed.

SQL> DECLARE
  2  s VARCHAR2(200);
  3  TYPE rcur IS REF CURSOR;      
  4  cur rcur; 
  5  writestring VARCHAR2(30);
  6  
  7  BEGIN
  8  s:='SELECT order_number||''.''||(SELECT level FROM DUAL  CONNECT BY level=1) as a
  9      FROM oe_order_headers_all WHERE rownum<10';
 10  
 11  OPEN cur FOR s;
 12  
 13  LOOP
 14             FETCH cur INTO writestring;
 15  EXIT WHEN cur%NOTFOUND;       
 16  DBMS_OUTPUT.PUT_LINE(writestring);
 17  END LOOP;
 18  
 19  END;
 20  /
1.1
1.1
2.1
2.1
3.1
3.1
4.1
5.1
6.1

PL/SQL procedure successfully completed.

SQL>