SYS_CONNECT_BY_PATH

SYS_CONNECT_BY_PATH(column,'char')

obclient> CREATE TABLE tbl1(z_id INT,name VARCHAR2(20),n_id INT);
Query OK, 0 rows affected (0.03 sec)

obclient> INSERT INTO tbl1 VALUES(0,'ABC',''),
    ->                (1,'An',0),(2,'Bn',0),(3,'Cn',0),
    ->                (4,'A1',1),(5,'B1',2),(6,'C1',3),
    ->                (7,'C2',6),(8,'A2',4),(9,'B2',5),
    ->                (10,'A3',8),(11,'A4',10),(12,'B3',9);
Query OK, 13 rows affected (0.00 sec)
Records: 13  Duplicates: 0  Warnings: 0

obclient> COMMIT;
Query OK, 0 rows affected (0.00 sec)

obclient> SELECT * FROM tbl1;
+------+------+------+
 Z_ID  NAME  N_ID 
+------+------+------+
    0  ABC   NULL 
    1  An       0 
    2  Bn       0 
    3  Cn       0 
    4  A1       1 
    5  B1       2 
    6  C1       3 
    7  C2       6 
    8  A2       4 
    9  B2       5 
   10  A3       8 
   11  A4      10 
   12  B3       9 
+------+------+------+
13 rows in set (0.01 sec)

obclient> SELECT z_id,name,n_id,SYS_CONNECT_BY_PATH(name, '/') "Path"
          FROM tbl1 
          START WITH n_id IS NULL 
          CONNECT BY PRIOR n_id = z_id
          ORDER BY z_id;
+------+------+------+---------------------+
 Z_ID  NAME  N_ID  Path                
+------+------+------+---------------------+
    0  ABC   NULL  /ABC                
    1  An       0  /ABC/An             
    2  Bn       0  /ABC/Bn             
    3  Cn       0  /ABC/Cn             
    4  A1       1  /ABC/An/A1          
    5  B1       2  /ABC/Bn/B1          
    6  C1       3  /ABC/Cn/C1          
    7  C2       6  /ABC/Cn/C1/C2       
    8  A2       4  /ABC/An/A1/A2       
    9  B2       5  /ABC/Bn/B1/B2       
   10  A3       8  /ABC/An/A1/A2/A3    
   11  A4      10  /ABC/An/A1/A2/A3/A4 
   12  B3       9  /ABC/Bn/B1/B2/B3    
+------+------+------+---------------------+
13 rows in set