Find duplicate values in SQL Server

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 )