How to Delete Duplicate Records in MySQL Without Using Temporary Tables?

Deleting Duplicate Records from a MySQL Table without Temporary Tables

Maintaining data integrity in a MySQL database is crucial, and removing duplicate records is often a necessary task. Many solutions exist for this problem, but some may involve using temporary tables, which can be resource-intensive. This article presents alternative methods to delete duplicate records without resorting to temporary tables.

Using a Unique Index

If the table lacks a primary key column, you can create a unique index on the columns that uniquely identify a record. In this case, the columns member_id, quiz_num, question_num, and answer_num should constitute the unique index. By adding this index, the database will automatically enforce uniqueness, preventing duplicate records from being inserted in the future.

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

Using a Primary Key and a Subquery

Alternatively, you can add a primary key to the table and use a subquery to identify and delete duplicate records. To add a primary key, execute the following query:

ALTER TABLE `TableA`
ADD PRIMARY KEY (`id`);

Once the primary key is in place, you can use the following query to delete duplicate records:

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
);

This query first identifies the duplicate records using the subquery and then removes them from the table. The id column is used to ensure that at least one record is retained for each duplicate set.

Choosing the Appropriate Method

Both methods effectively remove duplicate records without temporary tables. The choice between them depends on the specific requirements and the structure of the table. If future insertions of duplicate records are a concern, creating a unique index is recommended. Otherwise, using a primary key and subquery can be an efficient solution for one-time duplicate removal.

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