The SQL OVER() clause - when and why is it useful?

Question

    USE AdventureWorks2008R2;
GO
SELECT SalesOrderID, ProductID, OrderQty
    ,SUM(OrderQty) OVER(PARTITION BY SalesOrderID) AS \'Total\'
    ,AVG(OrderQty) OVER(PAR         


        

Answer 1

The OVER clause when combined with PARTITION BY state that the preceding function call must be done analytically by evaluating the returned rows of the query. Think of it as an inline GROUP BY statement.

OVER (PARTITION BY SalesOrderID) is stating that for SUM, AVG, etc... function, return the value OVER a subset of the returned records from the query, and PARTITION that subset BY the foreign key SalesOrderID.

So we will SUM every OrderQty record for EACH UNIQUE SalesOrderID, and that column name will be called 'Total'.

It is a MUCH more efficient means than using multiple inline views to find out the same information. You can put this query within an inline view and filter on Total then.

SELECT ...,
FROM (your query) inlineview
WHERE Total < 200