Question

I am trying to delete duplicate records from a mysql database. With the below command, it will delete all duplicates and will keep one row. In my database there are 300,000 records and I expect some 100,000 rows are duplicates.

The duplicates need to be deleted by the below command but the problem is, I had given the command in the evening after 9 hours it is still running.

 DELETE n1 FROM tableA n1,tableA n2 WHERE n1.title= n2.title AND n1.id > n2.id

What is happening? Can anyone explain?

Answer 1

Trying:

select * from tableA as n1 join tableA as n2 on n1.title = n2.title AND n1.id > n2.id;

And to explain it: n1.title = n2.title does not use an index.

This query will be better:

delete from `t2` where `id` in (
    select cid from (
       select max(id) as cid from t2 group by title having count(*) > 1
    ) as c
);