MySQL Mastery: Conquering Duplicate Rows with DELETE JOIN and ROW_NUMBER()

Here are some additional points to consider:

This code removes duplicates based on all columns in the table my_table.

DELETE t1
FROM my_table t1
INNER JOIN my_table t2 ON t1.id = t2.id AND t1.id < t2.id;

Explanation:

• We use DELETE to delete rows.
• t1 and t2 are aliases for the my_table table used within the join.
• INNER JOIN identifies rows where id values match between t1 and t2.
• t1.id < t2.id ensures we only delete the older row (based on the id order).

ROW_NUMBER() function:

DELETE FROM my_table
WHERE row_number() OVER (PARTITION BY name, email ORDER BY id) > 1;

• DELETE FROM my_table specifies the table to modify.
• WHERE clause filters the rows to be deleted.
• ROW_NUMBER() assigns a unique number to each row within partitions defined by name and email (ordered by id).
• The WHERE condition row_number()... > 1 targets rows where the number isn't 1 (indicating duplicates).

Remember:

• Replace my_table with your actual table name.
• Modify the columns used in the examples (id, name, and email) to match your definition of unique rows.
• It's recommended to back up your table before running these DELETE statements.

This method involves creating a temporary table to store the unique rows and then replacing the original table with the filtered data.

CREATE TEMPORARY TABLE unique_data AS
SELECT DISTINCT *
FROM your_table;

TRUNCATE TABLE your_table;

INSERT INTO your_table
SELECT * FROM unique_data;

DROP TEMPORARY TABLE unique_data;

• This method first creates a temporary table unique_data.
• SELECT DISTINCT * FROM your_table selects all columns with duplicates removed using DISTINCT.
• TRUNCATE TABLE your_table removes all existing data from the original table.
• INSERT INTO your_table SELECT * FROM unique_data populates the original table with the unique entries from the temporary table.
• Finally, DROP TEMPORARY TABLE unique_data removes the temporary table.

Using GROUP BY:

This method uses the GROUP BY clause to group rows and then optionally keeps only the first occurrence (or applies an aggregate function).

DELETE FROM your_table
WHERE id NOT IN (
  SELECT MIN(id)
  FROM your_table
  GROUP BY column1, column2, ...
);

• This approach uses DELETE to remove rows.
• The subquery within WHERE utilizes GROUP BY to group rows based on specified columns (column1, column2, etc.).
• MIN(id) selects the minimum id within each group, effectively keeping the first occurrence.
• id NOT IN (...) identifies rows where the id doesn't match the minimum IDs from the groups, meaning they are duplicates.

Choosing the Right Method:

• DELETE JOIN is efficient for smaller tables and when you want to delete duplicates based on specific conditions.
• ROW_NUMBER() is flexible for defining unique rows based on various columns and ordering.
• Temporary Table is a good option for larger tables but might require more temporary storage space.
• GROUP BY is efficient for simple duplicate removal based on specific columns and can be combined with aggregate functions.