Trailing edge detection for analog tags

If Trailing is specified as the parameter in the edge detection extension, the only rows in the result set are those that are the first to fail the criteria in the WHERE clause (returned false) after a row successfully met the WHERE clause criteria (returned true).

The following query selects the first values of "SysTimeSec" from the History table to fail the Value criterion between 10:00 and 10:02 a.m. on December 2, 2001.

SELECT DateTime, Value

FROM History

WHERE TagName = 'SysTimeSec'

AND DateTime >= '2001-12-02 10:00:00'

AND DateTime <= '2001-12-02 10:02:00'

AND wwRetrievalMode = 'Cyclic'

AND wwResolution = 2000

AND Value >= 50

AND wwEdgeDetection = 'Trailing'

The query returns only the two values that were the first to fail the criteria in the WHERE clause for the time period specified:

Compare these results with the same query using no edge detection, as shown in Edge detection for analog tags. Notice that even though the original query returned ten recorded rows for each value, the edge detection only returns the first row recorded after the event criteria returned false.