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History of the "Dual" Table [message #322871] Mon, 26 May 2008 09:10
Messages: 3211
Registered: April 2005
Location: Heilbronn, Germany
Senior Member
I just wondered the other day why a table that always holds exactly ONE row and ONE column is actually called "DUAL", and I have found the answer, which I would like to share:

It actually started out with exactly TWO rows.

Quote: Chuck Weiss, Oracle

I created the DUAL table as an underlying object in the Oracle Data Dictionary. It was never meant to be seen itself, but instead used inside a view that was expected to be queried. The idea was that you could do a JOIN to the DUAL table and create two rows in the result for every one row in your table. Then, by using GROUP BY, the resulting join could be summarized to show the amount of storage for the DATA extent and for the INDEX extent(s). The name, DUAL, seemed apt for the process of creating a pair of rows from just one.

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