Friday, February 29, 2008

Finding duplication entry using SQL

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 )

Reference: http://www.petefreitag.com/item/169.cfm