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How does oneselect EVERY Nthrow from a table?

DifferencebetweenTRUNCATE,DELETE and DROPcommands

Can one retrieveonly the Nth row

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Home Oracle FAQ Knowledge Base SQL and PL/SQL SQL

How does one eliminate duplicates rows from a table?Submitted by admin on Wed, 2004-08-04 14:06

Choose one of the following queries to identify or remove duplicate rows from a table leaving only unique

records in the table:

Method 1:

Delete all rowids that is BIGGER than the SMALLEST rowid value (for a given key).

Method 2:

This method is usually faster. However, remember to recreate all indexes, constraints, triggers, etc. on the

table when done.

Method 3:

(contributed by Dennis Gurnick)

Note: One can eliminate N^2 unnecessary operations by creating an index on the joined fields in the inner

loop (no need to loop through the entire table on each pass by a record). This will speed-up the deletion

process.

Note 2: If you are comparing NOT-NULL columns, use the NVL function. Remember that NULL is not equal to

NULL. This should not be a problem as all key columns should be NOT NULL by definition.

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Submitted by William Robertson (not verified) on Mon, 2005-02-07 19:15.

What about:

SQL> DELETE FROM table_name A WHERE ROWID > (

2 SELECT min(rowid) FROM table_name B

3 WHERE A.key_values = B.key_values);

SQL> create table table_name2 as select distinct * from table_name1;

SQL> drop table table_name1;

SQL> rename table_name2 to table_name1;

SQL> delete from my_table t1

SQL> where exists (select 'x' from my_table t2

SQL> where t2.key_value1 = t1.key_value1

SQL> and t2.key_value2 = t1.key_value2

SQL> and t2.rowid > t1.rowid);

How does one escape special characters

when writing SQL queries?

up How does one get the time difference

between two date columns?

Page 1 of 3How does one eliminate duplicates rows from a table? | Oracle FAQ

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from a table?

Tuning a 'LIKE-clause' by usingOracle Text orReverse KeyIndexes

How does onedrop/ rename acolumns in atable?

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Remove duplicate rows

Submitted by srikanth M S (not verified) on Thu, 2006-02-23 13:16.

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Remove duplicate Rows (modified)

Submitted by Srikanth M S (not verified) on Fri, 2006-02-24 05:57.

This query removes rows based on the column names specified in the GROUP BY clause. If you specify onlyone column name it will remove all duplicate records for that column. If you want to delete exact replica's ofthe same row - use all the column names in the GROUP BY.

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Using dense_rank()

Submitted by Seshagiri Vaddadi (not verified) on Sun, 2006-03-19 01:08.

Another example using the dense_rank() function:

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Using dense_rank()

Submitted by Srinivasan (not verified) on Thu, 2006-08-17 23:21.

Col1 and col2 are the primary key columns. To provide the correct SQL to use alias name "ln" for f dense_rankin the statement:

DELETE table_name

WHERE rowid IN

( SELECT LEAD(rowid) OVER

(PARTITION BY key_values ORDER BY NULL)

FROM table_name );

delete from where rowid not in

( select min(rowid)

from exp group by column1..,column2,...column3..);

delete from

where rowid not in ( select min(rowid)

from

group by column1..,column2,...column3..)

delete from table_name

where rowid in

(select rn from

(select rowid rn, dense_rank() over (partition by col1, col2, ..order by rowid) from table_name ) where rn 1

)

Page 2 of 3How does one eliminate duplicates rows from a table? | Oracle FAQ

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deleting duplicate rows from a table

Submitted by sriram (not verified) on Fri, 2006-05-19 01:24.

This statement deletes rows from the emp table where duplicate values of last_name appear.

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This Command will delete all duplicate rows

Submitted by varmas424 on Fri, 2010-03-05 23:05.

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Another Option

Submitted by rajat2011 on Fri, 2011-06-17 00:25.

It will be helpful especially when deletion is based on the date column:

DELETE FROM Table t1 WHERE COLUMN_NAME < (SELECT MAX(COLUMN_NAME) FROM Table T2 WHERE T1.COL=T2.COL);

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delete from

where rowid in (select rn from (Select rowid rn,

dense_rank() over (partition by col1,col2.. order by rowid) ln

from )

where ln 1)

delete from emp e

where empno in(select empno from emp d

where d.last_name=e.last_name

minus

select empno from emp f

where f.last_name=e.last_name

and rownum=1)

delete from emp where ('Delete',sal) in (

select case when count(*)>1 then 'Delete' else 'No' end DeleteFlag ,sal from emp group by sal having count(*)>1)

Page 3 of 3How does one eliminate duplicates rows from a table? | Oracle FAQ

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