Problem using 'Regular expression' in order to split characters of a column when there is no delimiter between them

Question

I have a table with below structure:

create table TBL_TEST
(
  col_id   NUMBER,
  col_name VARCHAR2(500)
)

Some example data :

col_id | col_name   
-----------------
  1    | aetnap        
  2    | elppa       
  3    | ananab     
 

What I need to do is to split characters of column col_name for each col_id for example for col_id=1 we must have :

col_id | col_name   
-----------------
  1    | a
  1    | e
  1    | t
  1    | n
  1    | a
  1    | p

this query is fine when there is only one record in the table :

SELECT col_id, REGEXP_SUBSTR(col_name, '[a-z]{1}', 1, LEVEL) AS VAL
  FROM tbl_test t
CONNECT BY REGEXP_SUBSTR(col_name, '[a-z]{1}', 1, LEVEL) is not null

but as soon as I insert another record in the table (say col_id=2 and col_id=3) I can not have the desired result. I want to know two things:

1. Why is this query works fine for one record and it does not for more ?
2. what is the best way to split the characters when there is no delimiter(like , or . or *) between them?

Thanks in advance

Answer 1

1- Because with multiple rows, multiple paths exist to reach a level in the hierarchy

select lvl, count(*) from (
SELECT col_id, REGEXP_SUBSTR(col_name, '[a-z]{1}', 1, LEVEL) AS VAL, level lvl
  FROM tbl_test
CONNECT BY REGEXP_SUBSTR(col_name, '[a-z]{1}', 1, LEVEL) is not null
) group by rollup(lvl)
order by 1;

       LVL   COUNT(*)
---------- ----------
         1          3
         2          9
         3         27
         4         81
         5        243
         6        486
                  849

2- I am not saying it is the best, it is just one possible way to do it (add delimiters and use other tricks):

drop table tbl_test;
create table tbl_test
(
  col_id   number,
  col_name varchar2(500)
);

insert into tbl_test values (1, 'aetnap');
insert into tbl_test values (2, 'elppa');
insert into tbl_test values (3, 'ananab');
commit;

select 
  col_id, trim(column_value) val
from tbl_test,
  xmltable(trim(trailing ',' from regexp_replace(col_name, '(.)', '"\1",')));

COL_ID VAL
------ ---
     1 a  
     1 e  
     1 t  
     1 n  
     1 a  
     1 p  
     2 e  
     2 l  
     2 p  
     2 p  
     2 a  
     3 a  
     3 n  
     3 a  
     3 n  
     3 a  
     3 b 

Here you can find multiple methods for tokenizing strings:

https://lalitkumarb.wordpress.com/2015/03/04/split-comma-delimited-strings-in-a-table-in-oracle/