How do I delete all the duplicate records in a MySQL table without temp tables
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?
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
);
Instead of drop table TableA
, you could delete all registers (delete from TableA;
) and then populate original table with registers coming from TableA_Verify (insert into TAbleA select * from TAbleA_Verify
). In this way you won't lost all references to original table (indexes,... )
CREATE TABLE TableA_Verify AS SELECT DISTINCT * FROM TableA;
DELETE FROM TableA;
INSERT INTO TableA SELECT * FROM TAbleA_Verify;
DROP TABLE TableA_Verify;
This doesn't use TEMP Tables, but real tables instead. If the problem is just about temp tables and not about table creation or dropping tables, this will work:
SELECT DISTINCT * INTO TableA_Verify FROM TableA;
DROP TABLE TableA;
RENAME TABLE TableA_Verify TO TableA;