How do you find duplicates in a table?

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Finding duplicates in a table is a common SQL task, usually done by identifying rows with the same values in specific columns. The approach typically involves GROUP BY with HAVING, or using window functions.

🔑 1. Using GROUP BY and HAVING

This method groups rows by the column(s) you want to check and filters groups having more than one record.

Example (find duplicate emails in users table):

SELECT email, COUNT(*) AS count
FROM users
GROUP BY email
HAVING COUNT(*) > 1;

✅ Returns only emails appearing more than once.

🔑 2. Using ROW_NUMBER() (Window Function)

You can assign row numbers within partitions of data and find duplicates easily.

SELECT id, name, email
FROM (
  SELECT id, name, email,
         ROW_NUMBER() OVER (PARTITION BY email ORDER BY id) AS row_num
  FROM users
) t
WHERE row_num > 1;

✅ Returns duplicate rows beyond the first occurrence.

🔑 3. Using DISTINCT and EXISTS

SELECT u1.*
FROM users u1
WHERE EXISTS (
  SELECT 1
  FROM users u2
  WHERE u1.email = u2.email
  AND u1.id <> u2.id
);

✅ Returns all rows that have at least one duplicate.

⚡ Key Notes

• Use GROUP BY + HAVING when you only need duplicate values.

• Use ROW_NUMBER() / RANK() when you want full details of duplicate rows.

• Use EXISTS / SELF JOIN for flexibility in complex queries.

👉 Summary:
To find duplicates, group records with GROUP BY + HAVING COUNT > 1, or use window functions for detailed duplicate rows. The best method depends on whether you need just the duplicate values or all duplicate rows.

Read More :

What is a JOIN? Explain types.

What is the difference between INNER JOIN and LEFT JOIN?

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