Finding Duplicates with SQL

Here's a query for finding duplicates in a table.

SELECT id_company,
  COUNT(id_company) AS occurrences
FROM company
GROUP BY id_company
HAVING ( COUNT(id_company) > 1 )

You could also use this technique to find rows that occur exactly once as bellow

SELECT id_company
FROM company
GROUP BY id_company
HAVING ( COUNT(id_company) = 1 )