EVERY determines
the execution cycle, that is, executes the query per time??’span minutes. The
other clauses are the same as in TinyDB. It can be seen from the query language
that Cougar does not support trigger operation. An example is given
below.
SELECT AVG(R.concentration)
FROM ChemicalSensor R
WHERE R.loc IN region
HAVING AVG(R.concentration)>0.6
DURATION (now, now+3600)
EVERY 10
The query is to detect whether the density of some chemical gas in a given
area exceeds the limit. The lifetime of the query is 3600 minutes from the
moment the query is submitted. The detecting is done per 10 minutes to see
if the density exceeds 0.6.
4 Storage and Index Techniques in Sensor Networks
For sensor networks, one of the most challenging problems is to name data.
Researchers have developed many data centric routing algorithms and communication
protocols [16, 17, 18, 8, 5]. Besides these algorithms and protocols,
sensor networks also need flexible data centric storage methods. For data centric
storage systems, every data generated by each sensor is stored at some
sensor(s) in the network according to its name. In the same way, it is easy to
find the corresponding data in the sensor network. This section talks about
the data centric storage and index techniques for sensor networks.
4.1 Data Centric Naming
There are many data naming approaches, and the naming scheme sometimes
depends on the applications. For example, some systems use hierarchical naming.
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