Find duplicate field values in MySQL

I recently had to clean up a database table of user data because it had the same email addresses assigned to multiple users. The first step was putting together a query that pulled only the duplicate email addresses and also told me how many times they occurred – from there the actual clean up process was fairly straight forward.

This simple query looked something like this:

SELECT email_address, COUNT(email_address) AS occurrences
FROM users
GROUP BY email_address
HAVING occurrences > 1

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I find this to be a particularly helpful SQL snippet as this kind of clean up process is fairly common. It uses MySQL’s HAVING clause because WHERE cannot handle aggregate functions – COUNT() in this case.

2 Thoughts

  1. Thanks for the snippet … but how can I get all records returned so I can compare the rows. In my case, duplicate records need to be compared manually, so I need more than just the count. Can you help with that?

  2. @dcolumbus, I have a suggestion. Sometimes the easiest thing to do is just export the table to Excel and then use Excel’s more intuitive tools to highlight the duplicate rows/values.

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