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196. Delete Duplicate Emails

Description

Table: Person

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| id          | int     |
| email       | varchar |
+-------------+---------+
id is the primary key (column with unique values) for this table.
Each row of this table contains an email. The emails will not contain uppercase letters.

 

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

For SQL users, please note that you are supposed to write a DELETE statement and not a SELECT one.

For Pandas users, please note that you are supposed to modify Person in place.

After running your script, the answer shown is the Person table. The driver will first compile and run your piece of code and then show the Person table. The final order of the Person table does not matter.

The result format is in the following example.

 

Example 1:

Input: 
Person table:
+----+------------------+
| id | email            |
+----+------------------+
| 1  | john@example.com |
| 2  | bob@example.com  |
| 3  | john@example.com |
+----+------------------+
Output: 
+----+------------------+
| id | email            |
+----+------------------+
| 1  | john@example.com |
| 2  | bob@example.com  |
+----+------------------+
Explanation: john@example.com is repeated two times. We keep the row with the smallest Id = 1.

Solutions

Delete the same email with a large Id.

Or, group according to the email, then use the Min keyword to pick out the smaller ones, and then delete the complement set.

Possible Pitfalls

p1.Id > p2.Id

here is >, not !=, since

  1. we are keeping smaller-id of duplicates in result
  2. if !=, then all duplicates will be removed, but we are expecting smallest-id duplicate being kept
  • import pandas as pd


# Modify Person in place
def delete_duplicate_emails(person: pd.DataFrame) -> None:
    # Sort the rows based on id (Ascending order)
    person.sort_values(by="id", ascending=True, inplace=True)
    # Drop the duplicates based on email.
    person.drop_duplicates(subset="email", keep="first", inplace=True)


  • # Write your MySQL query statement below
DELETE FROM Person
WHERE Id NOT IN (
    SELECT Id
    FROM (SELECT MIN(Id) Id FROM Person GROUP BY Email) p
    );

--

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

All Problems

All Solutions