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.
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