remove duplicate rows based on one column value

Question

I have the below table and now I need to delete the rows which are having duplicate \"refIDs\" but have atleast one row with that ref, i.e i need to remove row 4 and 5. plea

Answer 1

In MySQL, you can do this with a join in delete:

delete t
    from table t left join
         (select min(id) as id
          from table t
          group by refId
         ) tokeep
         on t.id = tokeep.id
    where tokeep.id is null;

For each RefId, the subquery calculates the minimum of the id column (presumed to be unique over the whole table). It uses a left join for the match, so anything that doesn't match has a NULL value for tokeep.id. These are the ones that are deleted.