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From Apache Wiki <wikidi...@apache.org>
Subject [Db-derby Wiki] Update of "OLAPOperations" by BryanPendleton
Date Thu, 19 Jul 2007 21:06:18 GMT
Dear Wiki user,

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The following page has been changed by BryanPendleton:
http://wiki.apache.org/db-derby/OLAPOperations

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     ROW_NUMBER () OVER (PARTITION BY department ORDER BY salary DESC)
  }}}
  
+ === Rolling-Up and Drilling-Down ===
+ 
+ People who are analyzing transaction or event data often wish to aggregate data at multiple
levels, such as providing totals, sub-totals, sub-sub-totals, etc. For example, you might
wish to  aggregate sales data by Territory, by Month, by Salesman, and by Product_line. This
is often called multi-dimensional aggregation, because each grouping attribute (Territory,
Month, Salesman, Product_line) provides a different dimension by which to analyze the data.
At different occasions in the analysis you may wish to aggregate larger amounts of data into
coarser groups, or you may wish to aggregate smaller amounts of data into finer groups. Reducing
the number of dimensions to aggregate at a coarser level is called '''rolling-up''' the data,
while increasing the number of dimensions to aggregate at a finer level is called '''drilling-down'''
the data.
+ 
+ With traditional SQL, each SELECT statement can only specify a single GROUP BY grouping.
The OLAP operations make it easier to specify these multi-dimensional aggregation queries.
In addition to being easier for the user to write, such queries can be run more efficiently
because the database can compute all the aggregates in a single scan of the data, rather than
one scan per grouping.
+ 

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