I need to remove duplicate rows from a fairly large SQL Server table (i.e. 300,000+ rows).
The rows, of course, will not be perfect duplicates because of the existence of the RowID identity field.
MyTable
RowID int not null identity(1,1) primary key,
Col1 varchar(20) not null,
Col2 varchar(2048) not null,
Col3 tinyint not null
How can I do this?
Assuming no nulls, you GROUP BY the unique columns, and SELECT the MIN (or MAX) RowId as the row to keep. Then, just delete everything that didn't have a row id:
DELETE FROM MyTable
LEFT OUTER JOIN (
SELECT MIN(RowId) as RowId, Col1, Col2, Col3
FROM MyTable
GROUP BY Col1, Col2, Col3
) as KeepRows ON
MyTable.RowId = KeepRows.RowId
WHERE
KeepRows.RowId IS NULL
In case you have a GUID instead of an integer, you can replace
MIN(RowId)
with
CONVERT(uniqueidentifier, MIN(CONVERT(char(36), MyGuidColumn)))
Another possible way of doing this is
;
--Ensure that any immediately preceding statement is terminated with a semicolon above
WITH cte
AS (SELECT ROW_NUMBER() OVER (PARTITION BY Col1, Col2, Col3
ORDER BY ( SELECT 0)) RN
FROM #MyTable)
DELETE FROM cte
WHERE RN > 1;
I am using ORDER BY (SELECT 0) above as it is arbitrary which row to preserve in the event of a tie.
To preserve the latest one in RowID order for example you could use ORDER BY RowID DESC
Execution Plans
The execution plan for this is often simpler and more efficient than that in the accepted answer as it does not require the self join.
This is not always the case however. One place where the GROUP BY solution might be preferred is situations where a hash aggregate would be chosen in preference to a stream aggregate.
The ROW_NUMBER solution will always give pretty much the same plan whereas the GROUP BY strategy is more flexible.
Factors which might favour the hash aggregate approach would be
No useful index on the partitioning columns
relatively fewer groups with relatively more duplicates in each group
In extreme versions of this second case (if there are very few groups with many duplicates in each) one could also consider simply inserting the rows to keep into a new table then TRUNCATE-ing the original and copying them back to minimise logging compared to deleting a very high proportion of the rows.
There's a good article on removing duplicates on the Microsoft Support site. It's pretty conservative - they have you do everything in separate steps - but it should work well against large tables.
I've used self-joins to do this in the past, although it could probably be prettied up with a HAVING clause:
DELETE dupes
FROM MyTable dupes, MyTable fullTable
WHERE dupes.dupField = fullTable.dupField
AND dupes.secondDupField = fullTable.secondDupField
AND dupes.uniqueField > fullTable.uniqueField
The following query is useful to delete duplicate rows. The table in this example has ID as an identity column and the columns which have duplicate data are Column1, Column2 and Column3.
DELETE FROM TableName
WHERE ID NOT IN (SELECT MAX(ID)
FROM TableName
GROUP BY Column1,
Column2,
Column3
/*Even if ID is not null-able SQL Server treats MAX(ID) as potentially
nullable. Because of semantics of NOT IN (NULL) including the clause
below can simplify the plan*/
HAVING MAX(ID) IS NOT NULL)
The following script shows usage of GROUP BY, HAVING, ORDER BY in one query, and returns the results with duplicate column and its count.
SELECT YourColumnName,
COUNT(*) TotalCount
FROM YourTableName
GROUP BY YourColumnName
HAVING COUNT(*) > 1
ORDER BY COUNT(*) DESC
delete t1
from table t1, table t2
where t1.columnA = t2.columnA
and t1.rowid>t2.rowid
Postgres:
delete
from table t1
using table t2
where t1.columnA = t2.columnA
and t1.rowid > t2.rowid
DELETE LU
FROM (SELECT *,
Row_number()
OVER (
partition BY col1, col1, col3
ORDER BY rowid DESC) [Row]
FROM mytable) LU
WHERE [row] > 1
This will delete duplicate rows, except the first row
DELETE
FROM
Mytable
WHERE
RowID NOT IN (
SELECT
MIN(RowID)
FROM
Mytable
GROUP BY
Col1,
Col2,
Col3
)
Refer (http://www.codeproject.com/Articles/157977/Remove-Duplicate-Rows-from-a-Table-in-SQL-Server)
I would prefer CTE for deleting duplicate rows from sql server table
strongly recommend to follow this article ::http://codaffection.com/sql-server-article/delete-duplicate-rows-in-sql-server/
by keeping original
WITH CTE AS
(
SELECT *,ROW_NUMBER() OVER (PARTITION BY col1,col2,col3 ORDER BY col1,col2,col3) AS RN
FROM MyTable
)
DELETE FROM CTE WHERE RN<>1
without keeping original
WITH CTE AS
(SELECT *,R=RANK() OVER (ORDER BY col1,col2,col3)
FROM MyTable)
DELETE CTE
WHERE R IN (SELECT R FROM CTE GROUP BY R HAVING COUNT(*)>1)
To Fetch Duplicate Rows:
SELECT
name, email, COUNT(*)
FROM
users
GROUP BY
name, email
HAVING COUNT(*) > 1
To Delete the Duplicate Rows:
DELETE users
WHERE rowid NOT IN
(SELECT MIN(rowid)
FROM users
GROUP BY name, email);
Quick and Dirty to delete exact duplicated rows (for small tables):
select distinct * into t2 from t1;
delete from t1;
insert into t1 select * from t2;
drop table t2;
I prefer the subquery\having count(*) > 1 solution to the inner join because I found it easier to read and it was very easy to turn into a SELECT statement to verify what would be deleted before you run it.
--DELETE FROM table1
--WHERE id IN (
SELECT MIN(id) FROM table1
GROUP BY col1, col2, col3
-- could add a WHERE clause here to further filter
HAVING count(*) > 1
--)
SELECT DISTINCT *
INTO tempdb.dbo.tmpTable
FROM myTable
TRUNCATE TABLE myTable
INSERT INTO myTable SELECT * FROM tempdb.dbo.tmpTable
DROP TABLE tempdb.dbo.tmpTable
I thought I'd share my solution since it works under special circumstances.
I my case the table with duplicate values did not have a foreign key (because the values were duplicated from another db).
begin transaction
-- create temp table with identical structure as source table
Select * Into #temp From tableName Where 1 = 2
-- insert distinct values into temp
insert into #temp
select distinct *
from tableName
-- delete from source
delete from tableName
-- insert into source from temp
insert into tableName
select *
from #temp
rollback transaction
-- if this works, change rollback to commit and execute again to keep you changes!!
PS: when working on things like this I always use a transaction, this not only ensures everything is executed as a whole, but also allows me to test without risking anything. But off course you should take a backup anyway just to be sure...
This query showed very good performance for me:
DELETE tbl
FROM
MyTable tbl
WHERE
EXISTS (
SELECT
*
FROM
MyTable tbl2
WHERE
tbl2.SameValue = tbl.SameValue
AND tbl.IdUniqueValue < tbl2.IdUniqueValue
)
it deleted 1M rows in little more than 30sec from a table of 2M (50% duplicates)
Using CTE. The idea is to join on one or more columns that form a duplicate record and then remove whichever you like:
;with cte as (
select
min(PrimaryKey) as PrimaryKey
UniqueColumn1,
UniqueColumn2
from dbo.DuplicatesTable
group by
UniqueColumn1, UniqueColumn1
having count(*) > 1
)
delete d
from dbo.DuplicatesTable d
inner join cte on
d.PrimaryKey > cte.PrimaryKey and
d.UniqueColumn1 = cte.UniqueColumn1 and
d.UniqueColumn2 = cte.UniqueColumn2;
Yet another easy solution can be found at the link pasted here. This one easy to grasp and seems to be effective for most of the similar problems. It is for SQL Server though but the concept used is more than acceptable.
Here are the relevant portions from the linked page:
Consider this data:
EMPLOYEE_ID ATTENDANCE_DATE
A001 2011-01-01
A001 2011-01-01
A002 2011-01-01
A002 2011-01-01
A002 2011-01-01
A003 2011-01-01
So how can we delete those duplicate data?
First, insert an identity column in that table by using the following code:
ALTER TABLE dbo.ATTENDANCE ADD AUTOID INT IDENTITY(1,1)
Use the following code to resolve it:
DELETE FROM dbo.ATTENDANCE WHERE AUTOID NOT IN (SELECT MIN(AUTOID) _
FROM dbo.ATTENDANCE GROUP BY EMPLOYEE_ID,ATTENDANCE_DATE)
This is the easiest way to delete duplicate record
DELETE FROM tblemp WHERE id IN
(
SELECT MIN(id) FROM tblemp
GROUP BY title HAVING COUNT(id)>1
)
Use this
WITH tblTemp as
(
SELECT ROW_NUMBER() Over(PARTITION BY Name,Department ORDER BY Name)
As RowNumber,* FROM <table_name>
)
DELETE FROM tblTemp where RowNumber >1
Here is another good article on removing duplicates.
It discusses why its hard: "SQL is based on relational algebra, and duplicates cannot occur in relational algebra, because duplicates are not allowed in a set."
The temp table solution, and two mysql examples.
In the future are you going to prevent it at a database level, or from an application perspective. I would suggest the database level because your database should be responsible for maintaining referential integrity, developers just will cause problems ;)
I had a table where I needed to preserve non-duplicate rows.
I'm not sure on the speed or efficiency.
DELETE FROM myTable WHERE RowID IN (
SELECT MIN(RowID) AS IDNo FROM myTable
GROUP BY Col1, Col2, Col3
HAVING COUNT(*) = 2 )
Oh sure. Use a temp table. If you want a single, not-very-performant statement that "works" you can go with:
DELETE FROM MyTable WHERE NOT RowID IN
(SELECT
(SELECT TOP 1 RowID FROM MyTable mt2
WHERE mt2.Col1 = mt.Col1
AND mt2.Col2 = mt.Col2
AND mt2.Col3 = mt.Col3)
FROM MyTable mt)
Basically, for each row in the table, the sub-select finds the top RowID of all rows that are exactly like the row under consideration. So you end up with a list of RowIDs that represent the "original" non-duplicated rows.
The other way is Create a new table with same fields and with Unique Index. Then move all data from old table to new table. Automatically SQL SERVER ignore (there is also an option about what to do if there will be a duplicate value: ignore, interrupt or sth) duplicate values. So we have the same table without duplicate rows. If you don't want Unique Index, after the transfer data you can drop it.
Especially for larger tables you may use DTS (SSIS package to import/export data) in order to transfer all data rapidly to your new uniquely indexed table. For 7 million row it takes just a few minute.
By useing below query we can able to delete duplicate records based on the single column or multiple column. below query is deleting based on two columns. table name is: testing and column names empno,empname
DELETE FROM testing WHERE empno not IN (SELECT empno FROM (SELECT empno, ROW_NUMBER() OVER (PARTITION BY empno ORDER BY empno)
AS [ItemNumber] FROM testing) a WHERE ItemNumber > 1)
or empname not in
(select empname from (select empname,row_number() over(PARTITION BY empno ORDER BY empno)
AS [ItemNumber] FROM testing) a WHERE ItemNumber > 1)
Create new blank table with the same structure
Execute query like this
INSERT INTO tc_category1
SELECT *
FROM tc_category
GROUP BY category_id, application_id
HAVING count(*) > 1
Then execute this query
INSERT INTO tc_category1
SELECT *
FROM tc_category
GROUP BY category_id, application_id
HAVING count(*) = 1
Another way of doing this :--
DELETE A
FROM TABLE A,
TABLE B
WHERE A.COL1 = B.COL1
AND A.COL2 = B.COL2
AND A.UNIQUEFIELD > B.UNIQUEFIELD
I would mention this approach as well as it can be helpful, and works in all SQL servers:
Pretty often there is only one - two duplicates, and Ids and count of duplicates are known. In this case:
SET ROWCOUNT 1 -- or set to number of rows to be deleted
delete from myTable where RowId = DuplicatedID
SET ROWCOUNT 0
From the application level (unfortunately). I agree that the proper way to prevent duplication is at the database level through the use of a unique index, but in SQL Server 2005, an index is allowed to be only 900 bytes, and my varchar(2048) field blows that away.
I dunno how well it would perform, but I think you could write a trigger to enforce this, even if you couldn't do it directly with an index. Something like:
-- given a table stories(story_id int not null primary key, story varchar(max) not null)
CREATE TRIGGER prevent_plagiarism
ON stories
after INSERT, UPDATE
AS
DECLARE #cnt AS INT
SELECT #cnt = Count(*)
FROM stories
INNER JOIN inserted
ON ( stories.story = inserted.story
AND stories.story_id != inserted.story_id )
IF #cnt > 0
BEGIN
RAISERROR('plagiarism detected',16,1)
ROLLBACK TRANSACTION
END
Also, varchar(2048) sounds fishy to me (some things in life are 2048 bytes, but it's pretty uncommon); should it really not be varchar(max)?
DELETE
FROM
table_name T1
WHERE
rowid > (
SELECT
min(rowid)
FROM
table_name T2
WHERE
T1.column_name = T2.column_name
);
CREATE TABLE car(Id int identity(1,1), PersonId int, CarId int)
INSERT INTO car(PersonId,CarId)
VALUES(1,2),(1,3),(1,2),(2,4)
--SELECT * FROM car
;WITH CTE as(
SELECT ROW_NUMBER() over (PARTITION BY personid,carid order by personid,carid) as rn,Id,PersonID,CarId from car)
DELETE FROM car where Id in(SELECT Id FROM CTE WHERE rn>1)
I you want to preview the rows you are about to remove and keep control over which of the duplicate rows to keep. See http://developer.azurewebsites.net/2014/09/better-sql-group-by-find-duplicate-data/
with MYCTE as (
SELECT ROW_NUMBER() OVER (
PARTITION BY DuplicateKey1
,DuplicateKey2 -- optional
ORDER BY CreatedAt -- the first row among duplicates will be kept, other rows will be removed
) RN
FROM MyTable
)
DELETE FROM MYCTE
WHERE RN > 1
So for each distinct value in a column of one table I want to insert that unique value into a row of another table.
list = select distinct(id) from table0
for distinct_id in list
insert into table1 (id) values (distinct_id)
end
Any ideas as to how to go about this?
Whenever you think about doing something in a loop, step back, and think again. SQL is optimized to work with sets. You can do this using a set-based query without the need to loop:
INSERT dbo.table1(id) SELECT DISTINCT id FROM dbo.table0;
There are some edge cases where looping can make more sense, but as SQL Server matures and more functionality is added, those edge cases get narrower and narrower...
insert into table1 (id)
select distinct id from table0
The following statement works with me.
insert into table1(col1, col2) select distinct on (col1) col1 col2 from table0
The below query will also check the existing data in the Table2.
INSERT INTO Table2(Id) SELECT DISTINCT Id FROM Table1 WHERE Id NOT IN(SELECT Id FROM Table2);
Other Simple way to copy distinct data with multiple columns from one table to other
Insert into TBL2
Select * from (Select COL1, ROW_NUMBER() over(PARTITION BY COL1 Order By COL1) AS COL2 From TBL1)T
where T.COL2 = 1
I have PostgreSQL 9 database that uses auto-incrementing integers as primary keys. I want to duplicate some of the rows in a table (based on some filter criteria), while changing one or two values, i.e. copy all column values, except for the ID (which is auto-generated) and possibly another column.
However, I also want to get the mapping from old to new IDs. Is there a better way to do it then just querying for the rows to copy first and then inserting new rows one at a time?
Essentially I want to do something like this:
INSERT INTO my_table (col1, col2, col3)
SELECT col1, 'new col2 value', col3
FROM my_table old
WHERE old.some_criteria = 'something'
RETURNING old.id, id;
However, this fails with ERROR: missing FROM-clause entry for table "old" and I can see why: Postgres must be doing the SELECT first and then inserting it and the RETURNING clauses only has access to the newly inserted row.
RETURNING can only refer to the columns in the final, inserted row. You cannot refer to the "OLD" id this way unless there is a column in the table to hold both it and the new id.
Try running this which should work and will show all the possible values that you can get via RETURNING:
INSERT INTO my_table (col1, col2, col3)
SELECT col1, 'new col2 value', col3
FROM my_table AS old
WHERE old.some_criteria = 'something'
RETURNING *;
It won't get you the behavior you want, but should illustrate better how RETURNING is designed to work.
This can be done with the help of data-modifiying CTEs (Postgres 9.1+):
WITH sel AS (
SELECT id, col1, col3
, row_number() OVER (ORDER BY id) AS rn -- order any way you like
FROM my_table
WHERE some_criteria = 'something'
ORDER BY id -- match order or row_number()
)
, ins AS (
INSERT INTO my_table (col1, col2, col3)
SELECT col1, 'new col2 value', col3
FROM sel
ORDER BY id -- redundant to be sure
RETURNING id
)
SELECT s.id AS old_id, i.id AS new_id
FROM (SELECT id, row_number() OVER (ORDER BY id) AS rn FROM ins) i
JOIN sel s USING (rn);
SQL Fiddle demonstration.
This relies on the undocumented implementation detail that rows from a SELECT are inserted in the order provided (and returned in the order provided). It works in all current versions of Postgres and is not going to break. Related:
Does Postgres preserve insertion order of records?
Window functions are not allowed in the RETURNING clause, so I apply row_number() in another subquery.
More explanation in this related later answer:
INSERT INTO ... FROM SELECT ... RETURNING id mappings
Good! I test this code, but I change
this (FROM my_table AS old) in (FROM my_table) and
this (WHERE old.some_criteria = 'something') in (WHERE some_criteria = 'something')
This is the final code that I use
INSERT INTO my_table (col1, col2, col3)
SELECT col1, 'new col2 value', col3
FROM my_table AS old
WHERE some_criteria = 'something'
RETURNING *;
Thanks!
DROP TABLE IF EXISTS tmptable;
CREATE TEMPORARY TABLE tmptable as SELECT * FROM products WHERE id = 100;
UPDATE tmptable SET id = sbq.id from (select max(id)+1 as id from products) as sbq;
INSERT INTO products (SELECT * FROM tmptable);
DROP TABLE IF EXISTS tmptable;
add another update before the insert to modify another field
UPDATE tmptable SET another = 'data';
'old' is a reserved word, used by the rule rewrite system.
[ I presume this query fragment is not part of a rule; in that case you would have phrased the question differently ]
I have a table with say 3 columns. There's no primary key so there can be duplicate rows. I need to just keep one and delete the others. Any idea how to do this is Sql Server?
I'd SELECT DISTINCT the rows and throw them into a temporary table, then drop the source table and copy back the data from the temp.
EDIT: now with code snippet!
INSERT INTO TABLE_2
SELECT DISTINCT * FROM TABLE_1
GO
DELETE FROM TABLE_1
GO
INSERT INTO TABLE_1
SELECT * FROM TABLE_2
GO
Add an identity column to act as a surrogate primary key, and use this to identify two of the three rows to be deleted.
I would consider leaving the identity column in place afterwards, or if this is some kind of link table, create a compound primary key on the other columns.
The following example works as well when your PK is just a subset of all table columns.
(Note: I like the approach with inserting another surrogate id column more. But maybe this solution comes handy as well.)
First find the duplicate rows:
SELECT col1, col2, count(*)
FROM t1
GROUP BY col1, col2
HAVING count(*) > 1
If there are only few, you can delete them manually:
set rowcount 1
delete from t1
where col1=1 and col2=1
The value of "rowcount" should be n-1 times the number of duplicates. In this example there are 2 dulpicates, therefore rowcount is 1. If you get several duplicate rows, you have to do this for every unique primary key.
If you have many duplicates, then copy every key once into anoher table:
SELECT col1, col2, col3=count(*)
INTO holdkey
FROM t1
GROUP BY col1, col2
HAVING count(*) > 1
Then copy the keys, but eliminate the duplicates.
SELECT DISTINCT t1.*
INTO holddups
FROM t1, holdkey
WHERE t1.col1 = holdkey.col1
AND t1.col2 = holdkey.col2
In your keys you have now unique keys. Check if you don't get any result:
SELECT col1, col2, count(*)
FROM holddups
GROUP BY col1, col2
Delete the duplicates from the original table:
DELETE t1
FROM t1, holdkey
WHERE t1.col1 = holdkey.col1
AND t1.col2 = holdkey.col2
Insert the original rows:
INSERT t1 SELECT * FROM holddups
btw and for completeness: In Oracle there is a hidden field you could use (rowid):
DELETE FROM our_table
WHERE rowid not in
(SELECT MIN(rowid)
FROM our_table
GROUP BY column1, column2, column3... ;
see: Microsoft Knowledge Site
Here's the method I used when I asked this question -
DELETE MyTable
FROM MyTable
LEFT OUTER JOIN (
SELECT MIN(RowId) as RowId, Col1, Col2, Col3
FROM MyTable
GROUP BY Col1, Col2, Col3
) as KeepRows ON
MyTable.RowId = KeepRows.RowId
WHERE
KeepRows.RowId IS NULL
This is a way to do it with Common Table Expressions, CTE. It involves no loops, no new columns or anything and won't cause any unwanted triggers to fire (due to deletes+inserts).
Inspired by this article.
CREATE TABLE #temp (i INT)
INSERT INTO #temp VALUES (1)
INSERT INTO #temp VALUES (1)
INSERT INTO #temp VALUES (2)
INSERT INTO #temp VALUES (3)
INSERT INTO #temp VALUES (3)
INSERT INTO #temp VALUES (4)
SELECT * FROM #temp
;
WITH [#temp+rowid] AS
(SELECT ROW_NUMBER() OVER (ORDER BY i ASC) AS ROWID, * FROM #temp)
DELETE FROM [#temp+rowid] WHERE rowid IN
(SELECT MIN(rowid) FROM [#temp+rowid] GROUP BY i HAVING COUNT(*) > 1)
SELECT * FROM #temp
DROP TABLE #temp
This is a tough situation to be in. Without knowing your particular situation (table size etc) I think that your best shot is to add an identity column, populate it and then delete according to it. You may remove the column later but I would suggest that you should keep it as it is really a good thing to have in the table
After you clean up the current mess you could add a primary key that includes all the fields in the table. that will keep you from getting into the mess again.
Of course this solution could very well break existing code. That will have to be handled as well.
Can you add a primary key identity field to the table?
Manrico Corazzi - I specialize in Oracle, not MS SQL, so you'll have to tell me if this is possible as a performance boost:-
Leave the same as your first step - insert distinct values into TABLE2 from TABLE1.
Drop TABLE1. (Drop should be faster than delete I assume, much as truncate is faster than delete).
Rename TABLE2 as TABLE1 (saves you time, as you're renaming an object rather than copying data from one table to another).
Here's another way, with test data
create table #table1 (colWithDupes1 int, colWithDupes2 int)
insert into #table1
(colWithDupes1, colWithDupes2)
Select 1, 2 union all
Select 1, 2 union all
Select 2, 2 union all
Select 3, 4 union all
Select 3, 4 union all
Select 3, 4 union all
Select 4, 2 union all
Select 4, 2
select * from #table1
set rowcount 1
select 1
while ##rowcount > 0
delete #table1 where 1 < (select count(*) from #table1 a2
where #table1.colWithDupes1 = a2.colWithDupes1
and #table1.colWithDupes2 = a2.colWithDupes2
)
set rowcount 0
select * from #table1
What about this solution :
First you execute the following query :
select 'set rowcount ' + convert(varchar,COUNT(*)-1) + ' delete from MyTable where field=''' + field +'''' + ' set rowcount 0' from mytable group by field having COUNT(*)>1
And then you just have to execute the returned result set
set rowcount 3 delete from Mytable where field='foo' set rowcount 0
....
....
set rowcount 5 delete from Mytable where field='bar' set rowcount 0
I've handled the case when you've got only one column, but it's pretty easy to adapt the same approach tomore than one column. Let me know if you want me to post the code.
How about:
select distinct * into #t from duplicates_tbl
truncate duplicates_tbl
insert duplicates_tbl select * from #t
drop table #t
I'm not sure if this works with DELETE statements, but this is a way to find duplicate rows:
SELECT *
FROM myTable t1, myTable t2
WHERE t1.field = t2.field AND t1.id > t2.id
I'm not sure if you can just change the "SELECT" to a "DELETE" (someone wanna let me know?), but even if you can't, you could just make it into a subquery.