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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