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How to Retrieve Duplicate Records in an SQL Table?

 
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Posts: 71
 Vinay Kumar
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Nov 09, 2024 6:24 am
(@Vinay Kumar)
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Joined: 4 years ago

Write an SQL query to find duplicate records based on a specific column in a table. Assume you have a customers table with columns customer_id, customer_name, and email. Find all customers with duplicate emails.

Answer:

To find duplicate records based on a specific column, such as email, we can use grouping with the HAVING clause. Let’s go through a solution that identifies duplicate emails in the customers table.

Solution: Using GROUP BY and HAVING

Assume the customers table structure is as follows:

customer_id customer_name email
1 John Doe john@example.com
2 Jane Smith jane@example.com
3 Johnathon Doe john@example.com
4 Emily Jones emily@example.com
5 Jane Austin jane@example.com

In this table, we want to identify all customers with duplicate email addresses.

SQL Query:

sql
 
SELECT email, COUNT(*) AS duplicate_count
FROM customers
GROUP BY email
HAVING COUNT(*) > 1;

Explanation:

  1. GROUP BY email: We use GROUP BY on the email column to aggregate rows with the same email.
  2. COUNT(*) for Duplicates: COUNT(*) counts the number of occurrences of each email.
  3. HAVING COUNT(*) > 1: The HAVING clause filters out records where the count is 1, leaving only emails that appear more than once.

This query returns each duplicate email along with the count of occurrences, helping us quickly identify and handle duplicate records.

Expected Output:

Based on the example data, the result would be:

email duplicate_count
john@example.com 2
jane@example.com 2

Extended Query: Retrieve Full Records of Duplicate Entries

If you want to retrieve the full records of customers with duplicate emails (e.g., to see customer_id and customer_name), you can use a JOIN with the initial query:

SQL Query:

sql
 
SELECT c.customer_id, c.customer_name, c.email
FROM customers c
JOIN (
SELECT email
FROM customers
GROUP BY email
HAVING COUNT(*) > 1
) AS duplicates
ON c.email = duplicates.email;

Explanation of Extended Query:

  1. Subquery for Duplicates: The inner query selects all duplicate emails using the method above.
  2. JOIN to Retrieve Full Records: We then join this result with the customers table to retrieve full records for each duplicate email.

Expected Output of Extended Query:

customer_id customer_name email
1 John Doe john@example.com
3 Johnathon Doe john@example.com
2 Jane Smith jane@example.com
5 Jane Austin jane@example.com

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