how to delete duplicate rows from a table in mysql

问题

I need to delete duplicate record from table in mysql. So i have a table name \"employee\" fields are empid, empname, empssn

for getting duplicate record i have written a query

     SELECT COUNT(empssn), empssn 
       FROM employee 
GROUP BY empssn 
    HAVING COUNT(empssn)>1

Now i want to delete duplicate records. for that i have written query is.

DELETE FROM employee 
          WHERE (empid, empssn) NOT IN (SELECT MIN(empid), empssn 
                                                           FROM employee 
                                                    GROUP BY empssn);

you can assume records in table are

EmpId  EmpName  EmpSSN
 -------------------------------
 1  Jack      555-55-5555
 2  Joe      555-56-5555
 3  Fred      555-57-5555
 4  Mike      555-58-5555
 5  Cathy      555-59-5555
 6  Lisa      555-70-5555
 7  Jack      555-55-5555
 8  Mike      555-58-5555
 9  Cathy      555-59-5555
10  Lisa      555-70-5555
11  Lisa      555-70-5555

but i have a mysql error is

You can\'t specify target table \'employee\' for update in FROM clause

回答1:

Does the trick of wrapping it into a derived table work for this case? (Based on http://www.xaprb.com/blog/2006/06/23/how-to-select-from-an-update-target-in-mysql/)

DELETE FROM employee WHERE (empid, empssn) NOT IN 
(
SELECT  empid, empssn FROM
(
SELECT MIN(empid) AS empid, empssn FROM employee GROUP BY empssn
) X
);

Edit Yep it seems to work this end.

来源：https://stackoverflow.com/questions/3271396/how-to-delete-duplicate-rows-from-a-table-in-mysql