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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
- we are keeping smaller-id of duplicates in result
- 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