Delta retrieval - query 1

As an example of how delta mode works, consider the following query:

SELECT TagName, DateTime, Value, QualityDetail

FROM History

WHERE TagName = 'A001'

AND DateTime >= '2009-09-12 00:20'

AND DateTime <= '2009-09-12 00:40'

AND wwRetrievalMode = 'Delta'

This query can be run against the following sample data:

A graphical representation of the data is as follows:

The results are:

The sample data points and the results are mapped on the following chart. Only the data falling between the time start and end marks at 2009-09-12 00:20 and 2009-09-12 00:40 (shown on the chart as dark vertical lines) are returned by the query.

Because there is no value that matches the start time, an initial value at 2009-09-12 00:20 is returned in the results based on the value of the preceding data point at 2009-09-12 00:16. Because there is no change in the value at 2009-09-12 00:27 from the value at 2009-09-12 00:24, the data point appears on the chart but does not appear in the results. Similarly, the second 0.0 value at 2009-09-12 00:29 is also excluded from the results.

You can further control the number of rows returned by using the wwTimeDeadband, wwValueDeadband, and wwCycleCount extensions. The use of a cycle count returns the first number of rows within the time range of the query. For more information, see -old-Using wwResolution, wwCycleCount, and wwRetrievalMode in the Same Query.

Also, the use of a time deadband and/or value deadband with delta retrieval produces differing results. For more information, see Time deadband (wwTimeDeadband) and Value deadband (wwValueDeadband).