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:
- Subquery to identify duplicate rows
- Use RANK to find and remove duplicate table rows
- Use self-join to remove duplicate rows
- Use analytics to detect and remove duplicate rows
- Delete duplicate table rows that contain NULL values
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
);
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
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
);
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
)
;
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
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
