[mysql] How to delete duplicates on a MySQL table?

I need to DELETE duplicated rows for specified sid on a MySQL table.

How can I do this with an SQL query?

DELETE (DUPLICATED TITLES) FROM table WHERE SID = "1"

Something like this, but I don't know how to do it.

This here will make the column column_name into a primary key, and in the meantime ignore all errors. So it will delete the rows with a duplicate value for column_name.

ALTER IGNORE TABLE `table_name` ADD PRIMARY KEY (`column_name`);

Suppose you have a table employee, with the following columns:

employee (first_name, last_name, start_date)

In order to delete the rows with a duplicate first_name column:

delete
from employee using employee,
    employee e1
where employee.id > e1.id
    and employee.first_name = e1.first_name  

This procedure will remove all duplicates (incl multiples) in a table, keeping the last duplicate. This is an extension of Retrieving last record in each group

Hope this is useful to someone.

DROP TABLE IF EXISTS UniqueIDs;
CREATE Temporary table UniqueIDs (id Int(11));

INSERT INTO UniqueIDs
    (SELECT T1.ID FROM Table T1 LEFT JOIN Table T2 ON
    (T1.Field1 = T2.Field1 AND T1.Field2 = T2.Field2 #Comparison Fields 
    AND T1.ID < T2.ID)
    WHERE T2.ID IS NULL);

DELETE FROM Table WHERE id NOT IN (SELECT ID FROM UniqueIDs);

I think this will work by basically copying the table and emptying it then putting only the distinct values back into it but please double check it before doing it on large amounts of data.

Creates a carbon copy of your table

create table temp_table like oldtablename; insert temp_table select * from oldtablename;

Empties your original table

DELETE * from oldtablename;

Copies all distinct values from the copied table back to your original table

INSERT oldtablename SELECT * from temp_table group by firstname,lastname,dob

Deletes your temp table.

Drop Table temp_table

You need to group by aLL fields that you want to keep distinct.

delete p from 
product p
inner join (
    select max(id) as id, url from product 
    group by url 
    having count(*) > 1
) unik on unik.url = p.url and unik.id != p.id;

Another easy way... using UPDATE IGNORE:

U have to use an index on one or more columns (type index). Create a new temporary reference column (not part of the index). In this column, you mark the uniques in by updating it with ignore clause. Step by step:

Add a temporary reference column to mark the uniques:

ALTER TABLE `yourtable` ADD `unique` VARCHAR(3) NOT NULL AFTER `lastcolname`;

=> this will add a column to your table.

Update the table, try to mark everything as unique, but ignore possible errors due to to duplicate key issue (records will be skipped):

UPDATE IGNORE `yourtable` SET `unique` = 'Yes' WHERE 1;

=> you will find your duplicate records will not be marked as unique = 'Yes', in other words only one of each set of duplicate records will be marked as unique.

Delete everything that's not unique:

DELETE * FROM `yourtable` WHERE `unique` <> 'Yes';

=> This will remove all duplicate records.

Drop the column...

ALTER TABLE `yourtable` DROP `unique`;

this removes duplicates in place, without making a new table

ALTER IGNORE TABLE `table_name` ADD UNIQUE (title, SID)

note: only works well if index fits in memory

You could just use a DISTINCT clause to select the "cleaned up" list (and here is a very easy example on how to do that).

This always seems to work for me:

CREATE TABLE NoDupeTable LIKE DupeTable; 
INSERT NoDupeTable SELECT * FROM DupeTable group by CommonField1,CommonFieldN;

Which keeps the lowest ID on each of the dupes and the rest of the non-dupe records.

I've also taken to doing the following so that the dupe issue no longer occurs after the removal:

CREATE TABLE NoDupeTable LIKE DupeTable; 
Alter table NoDupeTable Add Unique `Unique` (CommonField1,CommonField2);
INSERT IGNORE NoDupeTable SELECT * FROM DupeTable;

In other words, I create a duplicate of the first table, add a unique index on the fields I don't want duplicates of, and then do an Insert IGNORE which has the advantage of not failing as a normal Insert would the first time it tried to add a duplicate record based on the two fields and rather ignores any such records.

Moving fwd it becomes impossible to create any duplicate records based on those two fields.

Deleting duplicate rows in MySQL in-place, (Assuming you have a timestamp col to sort by) walkthrough:

Create the table and insert some rows:

create table penguins(foo int, bar varchar(15), baz datetime);
insert into penguins values(1, 'skipper', now());
insert into penguins values(1, 'skipper', now());
insert into penguins values(3, 'kowalski', now());
insert into penguins values(3, 'kowalski', now());
insert into penguins values(3, 'kowalski', now());
insert into penguins values(4, 'rico', now());
select * from penguins;
    +------+----------+---------------------+
    | foo  | bar      | baz                 |
    +------+----------+---------------------+
    |    1 | skipper  | 2014-08-25 14:21:54 |
    |    1 | skipper  | 2014-08-25 14:21:59 |
    |    3 | kowalski | 2014-08-25 14:22:09 |
    |    3 | kowalski | 2014-08-25 14:22:13 |
    |    3 | kowalski | 2014-08-25 14:22:15 |
    |    4 | rico     | 2014-08-25 14:22:22 |
    +------+----------+---------------------+
6 rows in set (0.00 sec)

Remove the duplicates in place:

delete a
    from penguins a
    left join(
    select max(baz) maxtimestamp, foo, bar
    from penguins
    group by foo, bar) b
    on a.baz = maxtimestamp and
    a.foo = b.foo and
    a.bar = b.bar
    where b.maxtimestamp IS NULL;
Query OK, 3 rows affected (0.01 sec)
select * from penguins;
+------+----------+---------------------+
| foo  | bar      | baz                 |
+------+----------+---------------------+
|    1 | skipper  | 2014-08-25 14:21:59 |
|    3 | kowalski | 2014-08-25 14:22:15 |
|    4 | rico     | 2014-08-25 14:22:22 |
+------+----------+---------------------+
3 rows in set (0.00 sec)

You're done, duplicate rows are removed, last one by timestamp is kept.

For those of you without a timestamp or unique column.

You don't have a timestamp or a unique index column to sort by? You're living in a state of degeneracy. You'll have to do additional steps to delete duplicate rows.

create the penguins table and add some rows

create table penguins(foo int, bar varchar(15)); 
insert into penguins values(1, 'skipper'); 
insert into penguins values(1, 'skipper'); 
insert into penguins values(3, 'kowalski'); 
insert into penguins values(3, 'kowalski'); 
insert into penguins values(3, 'kowalski'); 
insert into penguins values(4, 'rico'); 
select * from penguins; 
    # +------+----------+ 
    # | foo  | bar      | 
    # +------+----------+ 
    # |    1 | skipper  | 
    # |    1 | skipper  | 
    # |    3 | kowalski | 
    # |    3 | kowalski | 
    # |    3 | kowalski | 
    # |    4 | rico     | 
    # +------+----------+ 

make a clone of the first table and copy into it.

drop table if exists penguins_copy; 
create table penguins_copy as ( SELECT foo, bar FROM penguins );  

#add an autoincrementing primary key: 
ALTER TABLE penguins_copy ADD moo int AUTO_INCREMENT PRIMARY KEY first; 

select * from penguins_copy; 
    # +-----+------+----------+ 
    # | moo | foo  | bar      | 
    # +-----+------+----------+ 
    # |   1 |    1 | skipper  | 
    # |   2 |    1 | skipper  | 
    # |   3 |    3 | kowalski | 
    # |   4 |    3 | kowalski | 
    # |   5 |    3 | kowalski | 
    # |   6 |    4 | rico     | 
    # +-----+------+----------+ 

The max aggregate operates upon the new moo index:

delete a from penguins_copy a left join( 
    select max(moo) myindex, foo, bar 
    from penguins_copy 
    group by foo, bar) b 
    on a.moo = b.myindex and 
    a.foo = b.foo and 
    a.bar = b.bar 
    where b.myindex IS NULL; 

#drop the extra column on the copied table 
alter table penguins_copy drop moo; 
select * from penguins_copy; 

#drop the first table and put the copy table back: 
drop table penguins; 
create table penguins select * from penguins_copy; 

observe and cleanup

drop table penguins_copy; 
select * from penguins;
+------+----------+ 
| foo  | bar      | 
+------+----------+ 
|    1 | skipper  | 
|    3 | kowalski | 
|    4 | rico     | 
+------+----------+ 
    Elapsed: 1458.359 milliseconds 

What's that big SQL delete statement doing?

Table penguins with alias 'a' is left joined on a subset of table penguins called alias 'b'. The right hand table 'b' which is a subset finds the max timestamp [ or max moo ] grouped by columns foo and bar. This is matched to left hand table 'a'. (foo,bar,baz) on left has every row in the table. The right hand subset 'b' has a (maxtimestamp,foo,bar) which is matched to left only on the one that IS the max.

Every row that is not that max has value maxtimestamp of NULL. Filter down on those NULL rows and you have a set of all rows grouped by foo and bar that isn't the latest timestamp baz. Delete those ones.

Make a backup of the table before you run this.

Prevent this problem from ever happening again on this table:

If you got this to work, and it put out your "duplicate row" fire. Great. Now define a new composite unique key on your table (on those two columns) to prevent more duplicates from being added in the first place.

Like a good immune system, the bad rows shouldn't even be allowed in to the table at the time of insert. Later on all those programs adding duplicates will broadcast their protest, and when you fix them, this issue never comes up again.

This works for large tables:

 CREATE Temporary table duplicates AS select max(id) as id, url from links group by url having count(*) > 1;

 DELETE l from links l inner join duplicates ld on ld.id = l.id WHERE ld.id IS NOT NULL;

To delete oldest change max(id) to min(id)

Could it work if you count them, and then add a limit to your delete query leaving just one?

For example, if you have two or more, write your query like this:

DELETE FROM table WHERE SID = 1 LIMIT 1;

This work for me to remove old records:

delete from table where id in 
(select min(e.id)
    from (select * from table) e 
    group by column1, column2
    having count(*) > 1
); 

You can replace min(e.id) to max(e.id) to remove newest records.

I find Werner's solution above to be the most convenient because it works regardless of the presence of a primary key, doesn't mess with tables, uses future-proof plain sql, is very understandable.

As I stated in my comment, that solution hasn't been properly explained though. So this is mine, based on it.

1) add a new boolean column

alter table mytable add tokeep boolean;

2) add a constraint on the duplicated columns AND the new column

alter table mytable add constraint preventdupe unique (mycol1, mycol2, tokeep);

3) set the boolean column to true. This will succeed only on one of the duplicated rows because of the new constraint

update ignore mytable set tokeep = true;

4) delete rows that have not been marked as tokeep

delete from mytable where tokeep is null;

5) drop the added column

alter table mytable drop tokeep;

I suggest that you keep the constraint you added, so that new duplicates are prevented in the future.

here is how I usually eliminate duplicates

1. add a temporary column, name it whatever you want(i'll refer as active)
2. group by the fields that you think shouldn't be duplicate and set their active to 1, grouping by will select only one of duplicate values(will not select duplicates)for that columns
3. delete the ones with active zero
4. drop column active
5. optionally(if fits to your purposes), add unique index for those columns to not have duplicates again

DELETE T2
FROM   table_name T1
JOIN   same_table_name T2 ON (T1.title = T2.title AND T1.ID <> T2.ID)

delete from `table` where `table`.`SID` in 
    (
    select t.SID from table t join table t1 on t.title = t1.title  where t.SID > t1.SID
)

Deleting duplicates on MySQL tables is a common issue, that usually comes with specific needs. In case anyone is interested, here (Remove duplicate rows in MySQL) I explain how to use a temporary table to delete MySQL duplicates in a reliable and fast way, also valid to handle big data sources (with examples for different use cases).

Ali, in your case, you can run something like this:

-- create a new temporary table
CREATE TABLE tmp_table1 LIKE table1;

-- add a unique constraint    
ALTER TABLE tmp_table1 ADD UNIQUE(sid, title);

-- scan over the table to insert entries
INSERT IGNORE INTO tmp_table1 SELECT * FROM table1 ORDER BY sid;

-- rename tables
RENAME TABLE table1 TO backup_table1, tmp_table1 TO table1;

There are just a few basic steps when removing duplicate data from your table:

• Back up your table!
• Find the duplicate rows
• Remove the duplicate rows

Here is the full tutorial: https://blog.teamsql.io/deleting-duplicate-data-3541485b3473

If you want to keep the row with the lowest id value:

 DELETE n1 FROM 'yourTableName' n1, 'yourTableName' n2 WHERE n1.id > n2.id AND n1.email = n2.email

If you want to keep the row with the highest id value:

 DELETE n1 FROM 'yourTableName' n1, 'yourTableName' n2 WHERE n1.id < n2.id AND n1.email = n2.email

Here is a simple answer:

delete a from target_table a left JOIN (select max(id_field) as id, field_being_repeated  
    from target_table GROUP BY field_being_repeated) b 
    on a.field_being_repeated = b.field_being_repeated
      and a.id_field = b.id_field
    where b.id_field is null;

Love @eric's answer but it doesn't seem to work if you have a really big table (I'm getting The SELECT would examine more than MAX_JOIN_SIZE rows; check your WHERE and use SET SQL_BIG_SELECTS=1 or SET MAX_JOIN_SIZE=# if the SELECT is okay when I try to run it). So I limited the join query to only consider the duplicate rows and I ended up with:

DELETE a FROM penguins a
    LEFT JOIN (SELECT COUNT(baz) AS num, MIN(baz) AS keepBaz, foo
        FROM penguins
        GROUP BY deviceId HAVING num > 1) b
        ON a.baz != b.keepBaz
        AND a.foo = b.foo
    WHERE b.foo IS NOT NULL

The WHERE clause in this case allows MySQL to ignore any row that doesn't have a duplicate and will also ignore if this is the first instance of the duplicate so only subsequent duplicates will be ignored. Change MIN(baz) to MAX(baz) to keep the last instance instead of the first.

After running into this issue myself, on a huge database, I wasn't completely impressed with the performance of any of the other answers. I want to keep only the latest duplicate row, and delete the rest.

In a one-query statement, without a temp table, this worked best for me,

DELETE e.*
FROM employee e
WHERE id IN
 (SELECT id
   FROM (SELECT MIN(id) as id
          FROM employee e2
          GROUP BY first_name, last_name
          HAVING COUNT(*) > 1) x);

The only caveat is that I have to run the query multiple times, but even with that, I found it worked better for me than the other options.

The following works for all tables

CREATE TABLE `noDup` LIKE `Dup` ;
INSERT `noDup` SELECT DISTINCT * FROM `Dup` ;
DROP TABLE `Dup` ;
ALTER TABLE `noDup` RENAME `Dup` ;

Following remove duplicates for all SID-s, not only single one.

With temp table

CREATE TABLE table_temp AS
SELECT * FROM table GROUP BY title, SID;

DROP TABLE table;
RENAME TABLE table_temp TO table;

Since temp_table is freshly created it has no indexes. You'll need to recreate them after removing duplicates. You can check what indexes you have in the table with SHOW INDEXES IN table

Without temp table:

DELETE FROM `table` WHERE id IN (
  SELECT all_duplicates.id FROM (
    SELECT id FROM `table` WHERE (`title`, `SID`) IN (
      SELECT `title`, `SID` FROM `table` GROUP BY `title`, `SID` having count(*) > 1
    )
  ) AS all_duplicates 
  LEFT JOIN (
    SELECT id FROM `table` GROUP BY `title`, `SID` having count(*) > 1
  ) AS grouped_duplicates 
  ON all_duplicates.id = grouped_duplicates.id 
  WHERE grouped_duplicates.id IS NULL
)