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

Delete duplicate rows (duplicate data) in MySQL

Question:

How to remove duplicate rows from a MySQL table while retaining a single record for each unique value?

Example:

Consider the following data in the names table:

id	name
1	google
2	yahoo
3	msn
4	google
5	google
6	yahoo

Solution:

Warning: Always take the precaution of performing this operation on a test copy of the table first.

To remove all duplicate rows (keep one) and keep the record with the lowest or highest id value, use the following query:

Keep the lowest id value:

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

Keep the highest id value:

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

Additional notes:

• Make sure to add the condition AND n1.id != n2.id to both queries to prevent accidentally deleting all rows.
• This method is computationally expensive and may break your connection if the table is large.
• For large tables, consider using INSERT with DISTINCT as a faster alternative:

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

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