196. Delete Duplicate Emails | Easy
In this tutorial, we will tackle LeetCode SQL Problem 196, titled "Delete Duplicate Emails." This is a commonly asked SQL interview question, and it's rated as "Easy" on LeetCode. The objective of this problem is to identify and delete duplicate email entries from a table named Person, ensuring that we retain only one unique email with the smallest id.
Problem Explanation:
We are given a table Person that consists of two columns: id and email. The id is the primary key, and each row represents an email address. Some emails are duplicated, and our goal is to remove all duplicates while keeping only the record with the smallest id for each duplicate email. The tricky part is that we need to write a DELETE SQL statement, not a SELECT one.
SQL Approach:
In the tutorial, we will write an efficient SQL DELETE query to remove these duplicates. The approach revolves around leveraging a subquery to find duplicate emails and identify the records with larger id values that need to be deleted.
Here's a breakdown of the steps we’ll follow:
Identify Duplicate Emails: First, we need to find all the email addresses that appear more than once in the table.
Delete Extra Records: For each duplicate email, we will delete all records except the one with the smallest id.
Key Concepts Covered:
SQL DELETE Statements: You’ll learn how to use the DELETE statement to remove records from a table.
Subqueries: A crucial part of this problem is writing a subquery to find the ids of duplicate email records that need to be deleted.
Self-Join: We will explore how a self-join can be used to compare records in the same table.
Efficient SQL Queries: The tutorial will focus on writing efficient SQL that can handle large datasets.
Solution Outline:
To solve this, we will write a query that identifies the id of all records where the email is duplicated and the id is not the smallest for that email. After identifying these records, we will proceed to delete them.
Watch the full video for a step-by-step guide on how to implement this query, understand the logic behind it, and prepare yourself for SQL-related coding interviews.
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