SELECT DISTINCT CLOB_COLUMN FROM TABLE;

Question

I would like to find the distinct CLOB values that can assume the column called CLOB_COLUMN (of type CLOB) contained in the table called COPIA.

I have selected a PRO

Answer 1

To bypass the oracle error, you have to do something like this :

SELECT CLOB_COLUMN FROM TABLE COPIA C1 WHERE C1.ID IN (SELECT DISTINCT C2.ID FROM COPIA C2 WHERE ....)

Answer 2

If truncating the clob to the size of a varchar2 won't work, and you're worried about hash collisions, you can:

• Add a row number to every row;
• Use DBMS_lob.compare in a not exists subquery. Exclude duplicates (this means: compare = 0) with a higher rownum.

For example:

create table t (
  c1 clob
);

insert into t values ( 'xxx' );
insert into t values ( 'xxx' );
insert into t values ( 'yyy' );

commit;

with rws as (
   select row_number () over ( order by rowid ) rn,
          t.*
   from   t
)
  select c1 from rws r1
  where  not exists (
    select * from rws r2
    where  dbms_lob.compare ( r1.c1, r2.c1 ) = 0
    and    r1.rn > r2.rn
  );

C1    
xxx   
yyy  

Answer 3

add TO_CHAR after distinct keyword to convert CLOB to CHAR

SELECT DISTINCT TO_CHAR(CLOB_FIELD) from table1;   //This will return distinct values in CLOB_FIELD

Answer 4

You could compare the hashes of the CLOB to determine if they are different:

SELECT your_clob
  FROM your_table
 WHERE ROWID IN (SELECT MIN(ROWID) 
                   FROM your_table
                  GROUP BY dbms_crypto.HASH(your_clob, dbms_crypto.HASH_SH1))

---

Edit:

The HASH function doesn't guarantee that there will be no collision. By design however, it is really unlikely that you will get any collision. Still, if the collision risk (<2^80?) is not acceptable, you could improve the query by comparing (with dbms_lob.compare) the subset of rows that have the same hashes.

Answer 5

Use this approach. In table profile column content is NCLOB. I added the where clause to reduce the time it takes to run which is high,

with
  r as (select rownum i, content from profile where package = 'intl'),
  s as (select distinct (select min(i) from r where dbms_lob.compare(r.content, t.content) = 0) min_i from profile t where t.package = 'intl')
select (select content from r where r.i = s.min_i) content from s
;

It is not about to win any prizes for efficiency but should work.

Answer 6

select distinct DBMS_LOB.substr(column_name, 3000) from table_name;