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  | john@example.com |
| 2  | bob@example.com  |
| 3  | john@example.com |
+----+------------------+
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  | john@example.com |
| 2  | bob@example.com  |
+----+------------------+

Solution

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.

MySQL

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