How to remove duplicates from space separated list by Oracle regexp_replace? [duplicate]

Answer 1

Xquery?

select xmlquery('string-join(distinct-values(ora:tokenize(.," ")), " ")' passing  'A B A A C D' returning content) result  from dual

Answer 2

If I understand well you don't simply need to replace ',' with a space, but also to remove duplicates in a smarter way.

If I modify that expression to work with space instead of ',', I get

select regexp_replace('A B A A C D' ,'([^ ]+)( [ ]*\1)+', '\1') from dual

which gives 'A B A C D', not what you need.

A way to get your needed result could be the following, a bit more complicated:

with string(s) as ( select 'A B A A C D' from dual)    
    select listagg(case when rn = 1 then str end, ' ') within group (order by lev)
    from (
            select str,  row_number() over (partition by str order by 1) rn, lev
            from (
                SELECT trim(regexp_substr(s, '[^ ]+', 1, level)) str,
                       level as lev
                  FROM string
                CONNECT BY instr(s, ' ', 1, level - 1) > 0
                )
         )

My main problem here is that I'm not able to build a regexp that checks for non adjacent duplicates, so I need to split the string, check for duplicates and then aggregate again the non duplicated values, keeping the order.

If you don't mind the order of the tokens in the result string, this can be simplified:

with string(s) as ( select 'A B A A C D' from dual)
select listagg(str, ' ') within group (order by 1)
from (
        SELECT distinct trim(regexp_substr(s, '[^ ]+', 1, level)) as str
          FROM string
        CONNECT BY instr(s, ' ', 1, level - 1) > 0
     )

Answer 3

Assuming you want to keep the component strings in the order of their first occurrence (and not, say, reorder them alphabetically - your example is poorly chosen in this regard, because both lead to the same result), the problem is more complicated, because you must keep track of order too. Then for each letter you must keep just the first occurrence - here is where row_number() helps.

with
     inputs ( str ) as ( select 'A B A A C D' from dual)
-- end test data; solution begins below this line
select listagg(token, ' ') within group (order by id) as new_str
from (
       select level as id, regexp_substr(str, '[^ ]+', 1, level) as token,
              row_number() over ( 
                             partition by regexp_substr(str, '[^ ]+', 1, level)
                             order by level ) as rn

       from   inputs
       connect by regexp_substr(str, '[^ ]+', 1, level) is not null
     )
where rn = 1
;