196. Delete Duplicate Emails

Write a SQL query to delete all duplicate email entries in a table named Person, keeping only unique emails based on its smallest Id

+----+------------------+
| Id | Email            |
+----+------------------+
| 1  | [email protected] |
| 2  | [email protected]  |
| 3  | [email protected] |
+----+------------------+
Id is the primary key column for this table.

For example, after running your query, the above Person table should have the following rows:

+----+------------------+
| Id | Email            |
+----+------------------+
| 1  | [email protected] |
| 2  | [email protected]  |
+----+------------------+

SQL Schema

DROP TABLE IF EXISTS `Person`;

CREATE TABLE `Person`(
  Id INT NOT NULL AUTO_INCREMENT,
  Email VARCHAR(255),
  PRIMARY KEY (Id)
)ENGINE InnoDB CHARSET UTF8;

INSERT INTO `Person` VALUES (1, "[email protected]"), (2, "[email protected]"), (3, "[email protected]");

Approach: Using DELETE and WHERE clause [Accepted]
Algorithm:
By joining this table with itself on the Email column, we can get the following code.

SELECT 
  p1.*
FROM
  Person p1, Person p2
WHERE 
  p1.Email = p2.Email
;

Then we need to find the bigger id having same email address with other records. So we can add a new condition to the WHERE clause like this.

SELECT
  p1.*
FROM
  Person p1, Person p2
WHERE
  p1.Email = p2.Email AND p1.Id > p2.Id
;

As we already get the records to be deleted, we can alter this statement to DELETE in the end.

DELETE 
  p1
FROM
  Person p1, Person p2
WHERE
  p1.Email = p2.Email AND p1.Id > p2.Id
;

Algorithm (By myself)

I usually like use left join or inner join

First:

SELECT * FROM Person p INNER JOIN Person p2 ON  p.Email = p2.Email AND  p.Id > p2.Id;

Finally:

DELETE p FROM Person p INNER JOIN Person p2 ON  p.Email = p2.Email AND  p.Id > p2.Id;

Result Set:

mysql> SELECT * FROM `Person`;
+----+------------------+
| Id | Email            |
+----+------------------+
|  1 | [email protected] |
|  2 | [email protected]  |
+----+------------------+
2 rows in set (0.01 sec)