Connect By skipping some ancestors

Question

If I have a hierarchy set up in a table like the following:
id parentID permission
1 null y
2 1 n
3 2 y
4 3 n
5 4 y
using a connect by statement I could get the tree structure like 1->2->3->4->5. My question is, what if due to permissions (or some criteria), someone should only be able to get certain rows, but i still want them in their tree structure linked to the nearest ancestor they have permissions to. So the structure for the sample table i put up top should be 1->3->5. This is because 1 is the nearest ancestor of 3 that has permission = 'y', so 3 is linked under 1. Same for 5, its nearest ancestor with permission = 'y' is 3, so they get linked. This is just a simple example, because obviously each node could have more than one child. Does anyone know how i could do this all with one statement?

Answer 1

First one seems to work OK, not sure about second, I got 97 rows when I tried it against EMP table - perhaps I made a mistake (are all the joins to t necessary?)
Figured you might do this using SYS_CONNECT_BY_PATH in place of subquery. Below is a 'yours vs. mine', which I found did about 5% of the Logical IO (which was nice).
Personal Oracle Database 10g Release 10.1.0.2.0 - Production
With the Partitioning, OLAP and Data Mining options
SQL> SELECT empno, ename, privs
  2  FROM   emp;
     EMPNO ENAME      P
      7369 SMITH      Y
      7499 ALLEN      Y
      7521 WARD       Y
      7566 JONES      N
      7654 MARTIN     Y
      7698 BLAKE      Y
      7782 CLARK      Y
      7788 SCOTT      Y
      7839 KING       Y
      7844 TURNER     Y
      7876 ADAMS      Y
      7900 JAMES      Y
      7902 FORD       Y
      7934 MILLER     Y
14 rows selected.
SQL> SET AUTOTRACE ON STATISTICS;
SQL> WITH rslt AS (
  2    SELECT c.empno, c.ename,
  3          (SELECT a.empno
  4           FROM   emp a
  5           WHERE  a.privs = 'Y'
  6           START WITH a.empno = c.mgr
  7           CONNECT BY PRIOR a.mgr = a.empno
  8           AND PRIOR a.privs != 'Y') AS nearest
  9    FROM   emp c
10    WHERE  c.privs = 'Y')
11  SELECT empno, LPAD (' ', (LEVEL - 1) * 2) || ename ename
12  FROM   rslt
13  START WITH empno = 7839
14  CONNECT BY PRIOR empno = nearest;
     EMPNO ENAME
      7839 KING
      7698   BLAKE
      7499     ALLEN
      7521     WARD
      7654     MARTIN
      7844     TURNER
      7900     JAMES
      7782   CLARK
      7934     MILLER
      7788   SCOTT
      7876     ADAMS
      7902   FORD
      7369     SMITH
13 rows selected.
Statistics
          2  recursive calls
          8  db block gets
       2011  consistent gets
          1  physical reads
        648  redo size
        698  bytes sent via SQL*Net to client
        508  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
         18  sorts (memory)
          0  sorts (disk)
         13  rows processed
SQL> SELECT empno, LPAD (' ', (LEVEL - 1) * 2) || ename ename
  2  FROM  (SELECT empno, ename, RTRIM (SUBSTR (path, 1,
  3                  INSTR (path, '/', -1)), '/') path
  4         FROM  (SELECT empno, ename, SYS_CONNECT_BY_PATH (
  5                       DECODE (privs, 'Y', empno), '/') path
  6                FROM   emp
  7                WHERE  privs = 'Y'
  8                START WITH empno = 7839
  9                CONNECT BY PRIOR empno = mgr))
10  START WITH empno = 7839
11  CONNECT BY PRIOR empno = SUBSTR (path, INSTR (path, '/', -1) + 1);
     EMPNO ENAME
      7839 KING
      7788   SCOTT
      7876     ADAMS
      7902   FORD
      7369     SMITH
      7698   BLAKE
      7499     ALLEN
      7521     WARD
      7654     MARTIN
      7844     TURNER
      7900     JAMES
      7782   CLARK
      7934     MILLER
13 rows selected.
Statistics
          1  recursive calls
          0  db block gets
        112  consistent gets
          0  physical reads
          0  redo size
        698  bytes sent via SQL*Net to client
        508  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
         40  sorts (memory)
          0  sorts (disk)
         13  rows processed
SQL>