Finding duplicate values in a SQL table

Question

It\'s easy to find duplicates with one field:

SELECT name, COUNT(email) 
FROM users
GROUP BY email
HAVING COUNT(email) > 1

So if we have

Answer 1

The most important thing here is to have the fastest function. Also indices of duplicates should be identified. Self join is a good option but to have a faster function it is better to first find rows that have duplicates and then join with original table for finding id of duplicated rows. Finally order by any column except id to have duplicated rows near each other.

SELECT u.*
FROM users AS u
JOIN (SELECT username, email
      FROM users
      GROUP BY username, email
      HAVING COUNT(*)>1) AS w
ON u.username=w.username AND u.email=w.email
ORDER BY u.email;