How get the T-SQL code to find duplicates?

Question

MS Access has a button to generate sql code for finding duplicated rows. I don\'t know if SQL Server 2005/2008 Managment Studio has this.

1. If it has, please

Answer 1

Another way one can do this is by joining a table on itself.

SELECT *
FROM dbo.TableA aBase
JOIN dbo.TableA aDupes ON aDupes.ColA = aBase.ColA AND
                          aDupes.ColB = aBase.ColB
WHERE aBase.Pkey < aDupes.Pkey

Note: The aBase.Pkey < aDupes.Pkey is there because joining a table against itself will create two rows per match since the condition will always be true twice.

In other words: If table aBase has a row equal to a row from aDupes (based on ColA and ColB), the reflection of that match will also be true - that aDupes has a row equal to a row aBase based on ColA and ColB. Therefore both of those matches will be returned in the result set.

Narrow this down/eliminate this reflection by arbitrarily picking all results where one of the tables has a lower key.

< or > doesn't matter, as long as the keys are different.

This also takes care of filtering out matches with a row upon itself because aBase.Pkey < aDupes.Pkey forces the primary keys to be different.