Microsoft SQL Server 2008 R2 Dokumentacja Pobierz pdf (Strona 179)

Application Development CHAPTER 8 159

Count windows are completely different from the other window types because the size of

the windows is variable. When you create windows, you provide a parameter n as a count of

events to fulll within a window. For example, assume n is 2 as shown in Figure 8-5. The rst

window starts when the rst event starts and ends when the second event starts, because a

count of 2 events fullls the specication. The second event also resets the counter to 1 and

starts a new window. The third event increments the counter to 2, which ends the second window.

Input

events

Time

Count

windows

(n=2)

FIGURE 8-5 Count windows

Aggregations

You cannot perform aggregation operations on event streams directly; instead you must rst

create a window to group data into periods of time that you can then aggregate. You then

create an aggregation as a method of the window and, for all aggregations except Count, use

a lambda expression to assign the result to a eld.

StreamInsight supports the following aggregation functions:

■

Avg

■

Sum

■

Min

■

Max

■

Count

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Microsoft SQL Server 2008 R2 Dokumentacja Strona 179