CSC-SY

In order to find 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 )

Hope that helped.