How Can I Efficiently Delete Duplicate Rows in MySQL While Keeping One Record?

MySQL Duplicate Row Removal Guide: Efficiently Retain Unique Records

This article explains how to remove duplicate data from a MySQL table and retain one record from each set of duplicate data. While it is possible to filter out unique values ​​using SELECT DISTINCT, this article focuses on how to achieve this using the DELETE command.

Important Tips:

Be sure to back up your tables before performing any delete operations to prevent accidental data loss.

Keep the record with the smallest ID:

To keep the row with the smallest ID among duplicates, run the following SQL statement:

DELETE n1 FROM names n1, names n2 WHERE n1.id > n2.id AND n1.name = n2.name;

Keep the record with the largest ID:

To retain the row with the largest ID among duplicates, run the following SQL statement:

DELETE n1 FROM names n1, names n2 WHERE n1.id < n2.id AND n1.name = n2.name;

Other notes:

Efficiency warning:

The above method is not efficient and may cause database connection overload.

Integrity requirements:

MUST contain the AND n1.id < n2.id (or AND n1.id > n2.id) clause. Ignoring this condition may cause all rows in the table to be deleted.

Alternative:

For better performance, consider using the following INSERT and DISTINCT queries:

INSERT INTO tempTableName (cellId, attributeId, entityRowId, value)
SELECT DISTINCT cellId, attributeId, entityRowId, value
FROM tableName;

You can then delete the original table and rename the temporary table. This method avoids direct deletion of data, thereby improving efficiency and security.

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