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Delete duplicate rows from Oracle tables


Oracle Tips by Burleson Consulting

Removing duplicate rows from Oracle tables with SQL can be very tricky, and there are several techniques for identifying and removing duplicate rows from tables:

 

Use subquery to delete duplicate rows

Here we see an example of using SQL to delete duplicate table rows using an SQL subquery to identify duplicate rows, manually specifying the join columns:

DELETE FROM
   table_name A
WHERE
  a.rowid >
   ANY (
     SELECT
        B.rowid
     FROM
        table_name B
     WHERE
        A.col1 = B.col1
     AND
        A.col2 = B.col2
        );

Use RANK to delete duplicate rows

This is an example of the RANK function to identify and remove duplicate rows from Oracle tables, which deletes all duplicate rows while leaving the initial instance of the duplicate row:

delete from $table_name where rowid in
  (
  select "rowid" from
     (select "rowid", rank_n from
         (select rank() over (partition by $primary_key order by rowid) rank_n, rowid as "rowid"
             from $table_name
             where $primary_key in
                (select $primary_key from $table_name
                  group by $all_columns
                  having count(*) > 1
                )
             )
         )
     where rank_n > 1
  )

One of the most important features of Oracle is the ability to detect and remove duplicate rows from a table. While many Oracle DBA place primary key referential integrity constraints on a table, many shops do not use RI because they need the flexibility.


Use self-join to delete duplicate rows

The most effective way to detect duplicate rows is to join the table against itself as shown below.

select
   book_unique_id,
   page_seq_nbr,
   image_key
from
   page_image a
where
   rowid >
     (select min(rowid) from page_image b
      where
         b.key1 = a.key1
      and
         b.key2 = a.key2
      and
         b.key3 = a.key3
      );



Please note that you must specify all of the columns that make the row a duplicate in the SQL where clause. Once you have detected the duplicate rows, you may modify the SQL statement to remove the duplicates as shown below:

delete from
   table_name a
where
   a.rowid >
   any (select b.rowid
   from
      table_name b
   where
      a.col1 = b.col1
   and
      a.col2 = b.col2
   )
;

Use analytics to delete duplicate rows

You can also detect and delete duplicate rows using Oracle analytic functions:



delete from
   customer
where rowid in
 (select rowid from
   (select
     rowid,
     row_number()
    over
     (partition by custnbr order by custnbr) dup
    from customer)
  where dup > 1);



As we see, there are several ways to detect and delete duplicate rows from Oracle tables


Reader Comments:

Removing duplicate table rows where rows have NULL values

Rob Arden states:  The tip on this page helped with removing duplicate rows from Oracle tables. I thought this might be useful so I'm passing it on: I needed to add a null check because this fails to remove dupe rows where the fields match on a null value.  So instead of the given:

delete from
   table_name a
where
   a.rowid >
   any (select b.rowid
   from
      table_name b
   where
      a.col1 = b.col1
   and
      a.col2 = b.col2
   )
;



I needed to do the following to remove all of the duplicate table rows:



delete from
   table_name a
where
   a.rowid >
   any (select b.rowid
   from
      table_name b
   where
      (a.col1 = b.col1 or (a.col1 is null and b.col1 is null))
   and
      (a.col2 = b.col2 or (a.col2 is null and b.col2 is null))
   )
;

 
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