How to delete duplicates on a MySQL table?

Question

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

How can I do this with an SQL query?

Answer 1

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.