How to Delete Duplicate Rows in SQL?

In this section, we learn different ways to delete duplicate rows in MySQL and Oracle. If the SQL table contains duplicate rows, then we have to remove the duplicate rows.

Preparing sample data

The script creates the table named contacts.

DROP TABLE IF EXISTS contacts;
CREATE TABLE contacts (
id INT PRIMARY KEY AUTO_INCREMENT,
first_name VARCHAR(30) NOT NULL,
last_name VARCHAR(25) NOT NULL,
email VARCHAR(210) NOT NULL,
age VARCHAR(22) NOT NULL
);

In the above table, we have inserted the following data.

INSERT INTO contacts (first_name,last_name,email,age)
VALUES ('Kavin','Peterson','kavin.peterson@verizon.net','21'),
('Nick','Jonas','nick.jonas@me.com','18'),
('Peter','Heaven','peter.heaven@google.com','23'),
('Michal','Jackson','michal.jackson@aol.com','22'),
('Sean','Bean','sean.bean@yahoo.com','23'),
('Tom ','Baker','tom.baker@aol.com','20'),
('Ben','Barnes','ben.barnes@comcast.net','17'),
('Mischa ','Barton','mischa.barton@att.net','18'),
('Sean','Bean','sean.bean@yahoo.com','16'),
('Eliza','Bennett','eliza.bennett@yahoo.com','25'),
('Michal','Krane','michal.Krane@me.com','25'),
('Peter','Heaven','peter.heaven@google.com','20'),
('Brian','Blessed','brian.blessed@yahoo.com','20');
('Kavin','Peterson','kavin.peterson@verizon.net','30'),

We execute the script to recreate test data after executing a DELETE statement.

The query returns data from the contacts table:

SELECT * FROM contacts
ORDER BY email;

id	first_name	last_name	Email	age
7	Ben	Barnes	ben.barnes@comcast.net	21
13	Brian	Blessed	brian.blessed@yahoo.com	18
10	Eliza	Bennett	eliza.bennett@yahoo.cm	23
1	Kavin	Peterson	kavin.peterson@verizon.net	22
14	Kavin	Peterson	kavin.peterson@verizon.net	23
8	Mischa	Barton	mischa.barton@att.net	20
11	Michal	Krane	michal.Krane@me.com	17
4	Michal	Jackson	Michal.jackson@aol.com	18
2	Nick	Jonas	nick.jonas@me.com	16
3	Peter	Heaven	Peter.heaven@google.com	25
12	Peter	Heaven	Peter.heaven@google.com	25
5	Sean	Bean	Sean.bean@yahoo.com	20
9	Sean	Bean	Sean.bean@yahoo.com	20
6	Tom	Baker	tom.baker@aol.com	30

The following SQL query returns the duplicate emails from the contact table:

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

email	COUNT(email)
kavin.peterson@verizon.net	2
Peter.heaven@google.com	2
Sean.bean@yahoo.com	2

We have three rows with duplicate emails.

(A) Delete duplicate rows with the DELETE JOIN statement

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

Output:

Query OK, three rows affected (0.10 sec)

Three rows had been deleted. We execute the query, given below to finds the duplicate emails from the table.

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

The query returns the empty set. To verify the data from the contacts table, execute the following SQL query:

SELECT * FROM contacts;

id	first_name	last_name	Email	age
7	Ben	Barnes	ben.barnes@comcast.net	21
13	Brian	Blessed	brian.blessed@yahoo.com	18
10	Eliza	Bennett	eliza.bennett@yahoo.cm	23
1	Kavin	Peterson	kavin.peterson@verizon.net	22
8	Mischa	Barton	mischa.barton@att.net	20
11	Micha	Krane	michal.Krane@me.com	17
4	Michal	Jackson	Michal.jackson@aol.com	18
2	Nick	Jonas	nick.jonas@me.com	16
3	Peter	Heaven	Peter.heaven@google.com	25
5	Sean	Bean	Sean.bean@yahoo.com	20
6	Tom	Baker	tom.baker@aol.com	30

The rows id's 9, 12, and 14 have been deleted. We use the below statement to delete the duplicate rows:

Execute the script for creating the contact.

DELETE c1 FROM contacts c1
INNERJ OIN contacts c2
WHERE
c1.id > c2.id AND
c1.email = c2.email;

id	first_name	last_name	email	age
1	Ben	Barnes	ben.barnes@comcast.net	21
2	Kavin	Peterson	kavin.peterson@verizon.net	22
3	Brian	Blessed	brian.blessed@yahoo.com	18
4	Nick	Jonas	nick.jonas@me.com	16
5	Michal	Krane	michal.Krane@me.com	17
6	Eliza	Bennett	eliza.bennett@yahoo.cm	23
7	Michal	Jackson	Michal.jackson@aol.com	18
8	Sean	Bean	Sean.bean@yahoo.com	20
9	Mischa	Barton	mischa.barton@att.net	20
10	Peter	Heaven	Peter.heaven@google.com	25
11	Tom	Baker	tom.baker@aol.com	30

(B) Delete duplicate rows using an intermediate table

To delete a duplicate row by using the intermediate table, follow the steps given below:

Step 1. Create a new table structure, same as the real table:

CREATE TABLE source_copy LIKE source;

Step 2. Insert the distinct rows from the original schedule of the database:

INSERT INTO source_copy
SELECT * FROM source
GROUP BY col;

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 the 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: The ROW_NUMBER() function has been supported since MySQL version 8.02, so we should check our MySQL version before using the function.

The following statement uses the ROW_NUMBER () to assign a sequential integer to every row. If the email is duplicate, the row will higher than one.

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

The following SQL query 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;

Output:

Delete Duplicate Records in Oracle

When we found the duplicate records in the table, we had to delete the unwanted copies to keep our data clean and unique. If a table has duplicate rows, we can delete it by using the DELETE statement.

In the case, we have a column, which is not the part of group used to evaluate the duplicate records in the table.

Consider the table given below:

VEGETABLE_ID	VEGETABLE_NAME	COLOR
01	Potato	Brown
02	Potato	Brown
03	Onion	Red
04	Onion	Red
05	Onion	Red
06	Pumpkin	Green
07	Pumpkin	Yellow

-- create the vegetable table
CREATE TABLE vegetables (
VEGETABLE_ID NUMBER generated BY DEFAULT AS ID ENTITY,
VEGETABLE_NAME VARCHAR2(100),
color VARCHAR2(20),
PRIMARY KEY (VEGETABLE_ID)
);

-- insert sample rows
INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES('Potato','Brown');
INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES('Potato','Brown');
INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES('Onion','Red');
INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES('Onion','Red');
INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES('Onion','Red');
INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES('Pumpkin','Green');
INSERT INTO vegetables (VEGETABLE_NAME,color) VALUES('Pumpkin','Yellow');

-- query data from the vegetable table
SELECT * FROM vegetables;

Suppose, we want to keep the row with the highest VEGETABLE_ID and delete all other copies.

SELECT
MAX (VEGETABLE_ID)
FROM
vegetables
GROUP BY
VEGETABLE_NAME,
color
ORDER BY
MAX(VEGETABLE_ID);

MAX(VEGETABLE_ID)

We use the DELETE statement to delete the rows whose values in the VEGETABLE_ID COLUMN are not the highest.

DELETE FROM
vegetables
WHERE
VEGETABLE_IDNOTIN
(
SELECT
MAX(VEGETABLE_ID)
FROM
vegetables
GROUP BY
VEGETABLE_NAME,
color
);

Three rows have been deleted.

SELECT *FROM vegetables;

VEGETABLE_ID	VEGETABLE_NAME	COLOR
02	Potato	Brown
05	Onion	Red
06	Pumpkin	Green
07		Yellow

If we want to keep the row with the lowest id, use the MIN() function instead of the MAX() function.

DELETE FROM
vegetables
WHERE
VEGETABLE_IDNOTIN
(
SELECT
MIN(VEGETABLE_ID)
FROM
vegetables
GROUP BY
VEGETABLE_NAME,
color
);

The above method works if we have a column that is not part of the group for evaluating duplicate. If all values in the columns have copies, then we cannot use the VEGETABLE_ID column.

Let's drop and create the vegetable table with a new structure.

DROP TABLE vegetables;
CREATE TABLE vegetables (
VEGETABLE_ID NUMBER,
VEGETABLE_NAME VARCHAR2(100),
Color VARCHAR2(20)
);

INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color) VALUES(1,'Potato','Brown');
INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color) VALUES(1, 'Potato','Brown');
INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color)VALUES(2,'Onion','Red');
INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color)VALUES(2,'Onion','Red');
INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color) VALUES(2,'Onion','Red');
INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color) VALUES(3,'Pumpkin','Green');
INSERT INTO vegetables (VEGETABLE_ID,VEGETABLE_NAME,color) VALUES('4,Pumpkin','Yellow');

SELECT * FROM vegetables;

VEGETABLE_ID	VEGETABLE_NAME	COLOR
01	Potato	Brown
01	Potato	Brown
02	Onion	Red
02	Onion	Red
02	Onion	Red
03	Pumpkin	Green
04	Pumpkin	Yellow

In the vegetable table, the values in all columns VEGETABLE_ID, VEGETABLE_NAME, and color have been copied.

We can use the rowid, a locator that specifies where Oracle stores the row. Because the rowid is unique so that we can use it to remove the duplicates rows.

DELETE
FROM
Vegetables
WHERE
rowed NOT IN
(
SELECT
MIN(rowid)
FROM
vegetables
GROUP BY
VEGETABLE_ID,
VEGETABLE_NAME,
color
);

The query verifies the deletion operation:

SELECT * FROM vegetables;

VEGETABLE_ID	VEGETABLE_NAME	COLOR
01	Potato	Brown
02	Onion	Red
03	Pumpkin	Green
04	Pumpkin	Yellow

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