How to delete duplicate rows from a MySQL table

I have a MySQL table like:

ID, Col1, Col2, Col3, Col4, etc...

ID is a primary key and has been working since the table's creation.

What I want to do is delete all but one records where all the other columns are identical.

DELETE DupRows.*
FROM MyTable AS DupRows
   INNER JOIN (
      SELECT MIN(ID) AS minId, col1, col2
      FROM MyTable
      GROUP BY col1, col2
      HAVING COUNT(*) > 1
   ) AS SaveRows ON SaveRows.col1 = DupRows.col1 AND SaveRows.col2 = DupRows.col2
      AND SaveRows.minId <> DupRows.ID;

Of course you have to extend col1, col2 in all three places to all columns.

Edit: I just pulled this out of a script I keep and re-tested, it executes in MySQL.

user3768826

RENAME TABLE [table w/ duplicates] TO [temporary table name]
Create an identical table with the original table name which contained the duplicates.
INSERT INTO [new table] SELECT DISTINCT * FROM [old table with duplicates]
Delete the temporary tables.

Without nested selects or temporary tables.

DELETE  t1
FROM    table_name t1, table_name t2
WHERE   
            (t1.Col1 = t2.Col1 OR t1.Col1 IS NULL AND t2.Col1 IS NULL)
        AND (t1.Col2 = t2.Col2 OR t1.Col2 IS NULL AND t2.Col2 IS NULL)
        AND (t1.Col3 = t2.Col3 OR t1.Col3 IS NULL AND t2.Col3 IS NULL)
        AND (t1.Col4 = t2.Col4 OR t1.Col4 IS NULL AND t2.Col4 IS NULL)
        ...
        AND t1.ID < t2.ID;

I'd do it following way, in MSSQL, but I think it should work with slight modifications in MySQL. Not executable, but should show the way.

CREATE TEMPORARY TABLE #Table (Col1, Col2, Col3);
INSERT INTO #Table (Col1, Col2, Col3) SELECT DISTINCT Col1, Col2, Col3 FROM Table;
DELETE FROM Table;
INSERT INTO Table (Col1, Col2, Col3) SELECT Col1, Col2, Col3 FROM #Table;
DROP TABLE #Table;

you can also do this

Create table new_table{id, col1,col2,col3}

insert into new_table values(select distinct * from old_table)

drop table old_table

you can delete all the rows except one by using some function like Min(depends on db). Ex:

delete from Table_Name
where Id not in
( select min(Id)
from  Table_Name
group by ID, Col1, Col2, Col3, Col4);

You can try this with the help of join : Like that way:

DELETE e1 FROM emp_tbl AS e1 JOIN emp_tbl AS e2 WHERE 
e1.Col1=e2.Col1 AND e1.Col2=e2.Col2 AND e1.Col3=e2.Col3 AND e1.Col4=e2.Col4
AND e1.id < e2.id;

You can run an alter query and achieve this:

ALTER IGNORE TABLE tbl_1
ADD UNIQUE INDEX unq_idx(col1, col2, col3);

I cant guarantee it will retain the first record among the duplicates, but MySQL usually does that.

来源：https://stackoverflow.com/questions/5770228/how-to-delete-duplicate-rows-from-a-mysql-table

标签

mysql

duplicates