Here’s a handy query for finding duplicates in a table. Suppose you want to find all email addresses in a table that exist more than once:
SELECT email, COUNT(email) AS NumOccurrences FROM users GROUP BY email HAVING ( COUNT(email) > 1 )
You could also use this technique to find rows that occur exactly once:
SELECT email FROM users GROUP BY email HAVING ( COUNT(email) = 1 )
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