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Question

Formatted question description: https://leetcode.ca/all/196.html

Table: Person

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

 

Write an SQL query to delete all the duplicate emails, keeping only one unique email with the smallest id. Note that you are supposed to write a DELETE statement and not a SELECT one.

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 query 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.

Algorithm

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

Code

SQL

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

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