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Delete Duplicate Emails

Problem Description​

Write a SQL query to delete all the duplicate emails, keeping only one unique email with the smallest id.

Assume the table Person has the following structure:

+----+------------------+
| id | email            |
+----+------------------+
| 1  | john@example.com |
| 2  | bob@example.com  |
| 3  | john@example.com |
+----+------------------+

Example​

Example 1:

Assume that the `Person` table has the following content:
+----+------------------+
| id | email            |
+----+------------------+
| 1  | john@example.com |
| 2  | bob@example.com  |
| 3  | john@example.com |
+----+------------------+

Your query should delete the duplicate emails and the result table should look like:
+----+------------------+
| id | email            |
+----+------------------+
| 1  | john@example.com |
| 2  | bob@example.com  |
+----+------------------+

Constraints​

  • The Person table will have at most 10^4 rows.
  • Each email in the Person table will have at most 100 characters.

Solution Approach​

Approach Overview​

To delete duplicate emails while keeping the record with the smallest id, we can use a subquery to identify the duplicates and then delete them based on their id.

Detailed Steps​

  1. Identify Duplicates:
    • Use a subquery to find the smallest id for each duplicate email.
  2. Delete Duplicates:
    • Delete from the Person table where the id is not in the list of smallest ids for each email.

Code Examples​

SQL Query​

DELETE p1 FROM Person p1
JOIN Person p2
ON p1.email = p2.email
AND p1.id > p2.id;

Explanation​

  • The JOIN operation matches each row in the Person table with every other row having the same email.
  • The DELETE statement removes rows from the Person table where the id is greater than the smallest id for each email, effectively keeping only one unique email with the smallest id.

Complexity​

  • Time Complexity: O(n^2) in the worst case, where n is the number of rows in the Person table, due to the JOIN operation. However, for practical purposes with indexing and optimizations, the performance may be better.

  • Space Complexity: O(1), as no additional space is required beyond the query execution.