How to reverse the string 'ab,cd,ef' to 'ef->cd->ab' - sql

Question

when I select the table from Oracle, I want to handle one col'val :
eg:
'ab,cd,ef' to 'ef->cd->ab';
'AB,BC' to 'BC->AB';
'ACNN,BBCCAC' to 'BBCCAC->ACNN';
'BBBDC，DCCX，FFF' to 'FFF->DCCX->BBBDC'

Answer 1

We have two tasks. The first is to tokenize the original strings. This is quite easy with regular expressions (although there are more performant approaches if you are dealing with large volumes). The second task is to re-assemble the tokens in reverse order; we can use the 11gR2 LISTAGG() function for this:
with tokens as (
select distinct col1, regexp_substr(col1, '[^,]+', 1, level) as tkn, level as rn
from t23
connect by level <= regexp_count (col1, '[,]') +1
)
select col1
, listagg(tkn, '->')
within group (order by rn desc) as rev_col1
from tokens
group by col1
/
Here is a SQL Fiddle.

Answer 2

You can do it with a mix of string split and string aggregation.
Using:
REGEXP_SUBSTR : To split the comma delimited string into rows
LISTAGG : To aggregate the values
You can have a look at this article to understand how string split works http://lalitkumarb.wordpress.com/2015/03/04/split-comma-delimited-strings-in-a-table-using-oracle-sql/
SQL> WITH DATA AS(
2 SELECT 1 ID, 'ab,cd,ef' text FROM dual UNION ALL
3 SELECT 2 ID, 'AB,BC' text FROM dual UNION ALL
4 SELECT 3 ID, 'ACNN,BBCCAC' text FROM dual
5 )
6 SELECT ID,
7 listagg(text, ',') WITHIN GROUP (
8 ORDER BY rn DESC) reversed_indices
9 FROM
10 (SELECT t.id,
11 rownum rn,
12 trim(regexp_substr(t.text, '[^,]+', 1, lines.COLUMN_VALUE)) text
13 FROM data t,
14 TABLE (CAST (MULTISET
15 (SELECT LEVEL FROM dual CONNECT BY LEVEL <= regexp_count(t.text, ',')+1
16 ) AS sys.odciNumberList ) ) lines
17 ORDER BY ID
18 )
19 GROUP BY ID
20 /
ID REVERSED_INDICES
---------- ------------------------------
1 ef,cd,ab
2 BC,AB
3 BBCCAC,ACNN
SQL>
Let's say your table looks like:
SQL> SELECT * FROM t;
ID TEXT
---------- ------------------------------
1 ab,cd,ef
2 AB,BC
3 ACNN,BBCCAC
4 word1,word2,word3
5 1,2,3
SQL>
Using the above query:
SQL> SELECT ID,
2 listagg(text, '-->') WITHIN GROUP (
3 ORDER BY rn DESC) reversed_indices
4 FROM
5 (SELECT t.id,
6 rownum rn,
7 trim(regexp_substr(t.text, '[^,]+', 1, lines.COLUMN_VALUE)) text
8 FROM t,
9 TABLE (CAST (MULTISET
10 (SELECT LEVEL FROM dual CONNECT BY LEVEL <= regexp_count(t.text, ',')+1
11 ) AS sys.odciNumberList ) ) lines
12 ORDER BY ID
13 )
14 GROUP BY ID
15 /
ID REVERSED_INDICES
---------- ------------------------------
1 ef-->cd-->ab
2 BC-->AB
3 BBCCAC-->ACNN
4 word3-->word2-->word1
5 3-->2-->1
SQL>