https://www.mysqltutorial.org/mysql-delete-duplicate-rows/

How To Delete Duplicate Rows in MySQL

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Summary: in this tutorial, you will learn various ways to delete duplicate rows in MySQL.
In the previous tutorial, we have shown you how to find duplicate values in a table. Once the duplicates rows are identified, you may want to delete them to clean up your data.

Prepare sample data

The following script creates table contacts and inserts sample data into the contacts table for the demonstration.

        

            
        

DROP TABLE IF EXISTS contacts;

CREATE TABLE contacts (
    id INT PRIMARY KEY AUTO_INCREMENT,
    first_name VARCHAR(50) NOT NULL,
    last_name VARCHAR(50) NOT NULL, 
    email VARCHAR(255) NOT NULL
);

INSERT INTO contacts (first_name,last_name,email) 
VALUES ('Carine ','Schmitt','carine.schmitt@verizon.net'),
       ('Jean','King','jean.king@me.com'),
       ('Peter','Ferguson','peter.ferguson@google.com'),
       ('Janine ','Labrune','janine.labrune@aol.com'),
       ('Jonas ','Bergulfsen','jonas.bergulfsen@mac.com'),
       ('Janine ','Labrune','janine.labrune@aol.com'),
       ('Susan','Nelson','susan.nelson@comcast.net'),
       ('Zbyszek ','Piestrzeniewicz','zbyszek.piestrzeniewicz@att.net'),
       ('Roland','Keitel','roland.keitel@yahoo.com'),
       ('Julie','Murphy','julie.murphy@yahoo.com'),
       ('Kwai','Lee','kwai.lee@google.com'),
       ('Jean','King','jean.king@me.com'),
       ('Susan','Nelson','susan.nelson@comcast.net'),
       ('Roland','Keitel','roland.keitel@yahoo.com');

Note that you can execute this script to recreate test data after you execute a DELETE statement.
This query returns data from the contacts table:

        

            
        

SELECT * FROM contacts
ORDER BY email;

The following query returns the duplicate emails in the contacts table:

        

            
        

SELECT 
    email, COUNT(email)
FROM
    contacts
GROUP BY 
    email
HAVING 
    COUNT(email) > 1;

As you can see, we have four rows with duplicate emails.

A) Delete duplicate rows using DELETE JOIN statement

MySQL provides you with the DELETE JOIN statement that allows you to remove duplicate rows quickly.
The following statement deletes duplicate rows and keeps the highest id:

        

            
        

DELETE t1 FROM contacts t1
INNER JOIN contacts t2 
WHERE 
    t1.id < t2.id AND 
    t1.email = t2.email;

This query references the contacts table twice, therefore, it uses the table alias t1 and t2.
The output is:

Query OK, 4 rows affected (0.10 sec)

It indicated that four rows have been deleted. You can execute the query that find duplicate emails again to verify the delete:

        

            
        

SELECT 
    email, 
    COUNT(email)
FROM
    contacts
GROUP BY 
    email
HAVING 
    COUNT(email) > 1;

The query returns an empty set, which means that the duplicate rows have been deleted.
Let’s verify data from the contacts table:

        

            
        

SELECT * FROM contacts;

The rows with id 2, 4, 7, and 9 have been deleted.
In case you want to delete duplicate rows and keep the lowest id, you can use the following statement:

        

            
        

DELETE c1 FROM contacts c1
INNER JOIN contacts c2 
WHERE
    c1.id > c2.id AND 
    c1.email = c2.email;

Note that you can execute the script for creating contacts table again and test this query. The following output shows the data of the contacts table after removing duplicate rows.

B) Delete duplicate rows using an intermediate table

The following shows the steps for removing duplicate rows using an intermediate table:

1. Create a new table with the structure the same as the original table that you want to delete duplicate rows.
2. Insert distinct rows from the original table to the immediate table.
3. Drop the original table and rename the immediate table to the original table.

The following queries illustrate the steps:
Step 1. Create a new table whose structure is the same as the original table:

        

            
        

CREATE TABLE source_copy LIKE source;

Step 2. Insert distinct rows from the original table to the new table:

        

            
        

INSERT INTO source_copy
SELECT * FROM source
GROUP BY col; -- column that has duplicate values

Step 3. drop the original table and rename the immediate table to the original one

        

            
        

DROP TABLE source;
ALTER TABLE source_copy RENAME TO source;

For example, the following statements delete rows with duplicate emails from the contacts table:

        

            
        

-- step 1
CREATE TABLE contacts_temp 
LIKE contacts;

-- step 2
INSERT INTO contacts_temp
SELECT * 
FROM contacts 
GROUP BY email;


-- step 3
DROP TABLE contacts;

ALTER TABLE contacts_temp 
RENAME TO contacts;

C) Delete duplicate rows using the ROW_NUMBER() function

Note that the ROW_NUMBER() function has been supported since MySQL version 8.02 so you should check your MySQL version before using the function.

The following statement uses the ROW_NUMBER() function to assign a sequential integer number to each row. If the email is duplicate, the row number will be greater than one.

        

            
        

SELECT 
 id, 
    email, 
    ROW_NUMBER() OVER ( 
  PARTITION BY email 
        ORDER BY email
 ) AS row_num 
FROM contacts;

The following statement returns id list of the duplicate rows:

        

            
        

SELECT 
 id 
FROM (
 SELECT 
  id,
  ROW_NUMBER() OVER (
   PARTITION BY email
   ORDER BY email) AS row_num
 FROM 
  contacts
) t
WHERE 
 row_num > 1;

And you just delete the duplicate rows from the contacts table using the DELETE statement with a subquery in the WHERE clause:

        

            
        

DELETE FROM contacts 
WHERE 
 id IN (
 SELECT 
  id 
 FROM (
  SELECT 
   id,
   ROW_NUMBER() OVER (
    PARTITION BY email
    ORDER BY email) AS row_num
  FROM 
   contacts
  
 ) t
    WHERE row_num > 1
);

MySQL issued the following message:

4 row(s) affected

In this tutorial, you have learned how to delete duplicate rows in MySQL by using the the DELETE JOIN statement or an intermediate table.