Finding Duplicates with SQL

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 )