sql - How do I delete all the duplicate records in a MySQL table without temp tables

Question

I've seen a number of variations on this but nothing quite matches what I'm trying to accomplish.

I have a table, TableA, which contain the answers given by users to configurable questionnaires. The columns are member_id, quiz_num, question_num, answer_num.

Somehow a few members got their answers submitted twice. So I need to remove the duplicated records, but make sure that one row is left behind.

There is no primary column so there could be two or three rows all with the exact same data.

Is there a query to remove all the duplicates?

Answer 1

Add Unique Index on your table:

ALTER IGNORE TABLE `TableA`   
ADD UNIQUE INDEX (`member_id`, `quiz_num`, `question_num`, `answer_num`);

Another way to do this would be:

Add primary key in your table then you can easily remove duplicates from your table using the following query:

DELETE FROM member  
WHERE id IN (SELECT * 
             FROM (SELECT id FROM member 
                   GROUP BY member_id, quiz_num, question_num, answer_num HAVING (COUNT(*) > 1)
                  ) AS A
            );