суббота, 8 января 2011 г.

Finding Duplicates with 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 )
Via Huajun Zhai's Blog. Also see How to remove duplicate rows from a table - Microsoft Knowledge base article.

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