Tip: OVER and PARTITION BY

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Here’s a quick summary of OVER and PARTITION BY (new in SQL 2005), for the uninitiated or forgetful…

OVER

OVER allows you to get aggregate information without using a GROUP BY. In other words, you can retrieve detail rows, and get aggregate data alongside it. For example, this query:

SELECT SUM(Cost) OVER () AS Cost

, OrderNum

FROM Orders

Will return something like this:

Cost  OrderNum

10.00 345

10.00 346

10.00 347

10.00 348

Quick translation:

  • SUM(cost) – get me the sum of the COST column
  • OVER – for the set of rows….
  • () – …that encompasses the entire result set.

OVER(PARTITION BY)

OVER, as used in our previous example, exposes the entire resultset to the aggregation…”Cost” was the sum of all [Cost]  in the resultset.  We can break up that resultset into partitions with the use of PARTITION BY:

SELECT SUM(Cost) OVER (PARTITION BY CustomerNo) AS Cost

, OrderNum

, CustomerNo

FROM Orders

My partition is by CustomerNo – each “window” of a single customer’s orders will be treated separately from each other “window”….I’ll get the sum of cost for Customer 1, and then the sum for Customer 2:

Cost  OrderNum   CustomerNo

 8.00 345        1

 8.00 346        1

 8.00 347        1

 2.00 348        2

The translation here is:

  • SUM(cost) – get me the sum of the COST column
  • OVER – for the set of rows….
  • (PARTITION BY CustomerNo) – …that have the same CustomerNo.

 

Further Reading: BOL: OVER Clause

Enjoy, and happy days!

-Jen

http://www.MidnightDBA.com/Jen

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