Edge detection for analog tags

For example, the behavior of the WHERE clause as it processes a result set can be illustrated as:

Slopes A-B, C-D and E-F are rising edges, while slopes B-C, D-E and F-G are falling edges. The slopes are affected by the WHERE clause, which is a combination of the wwEdgeDetection option and the comparison operator used against the value.

The following table describes the rows that would be returned for the various edge detection settings:

* If the falling edge is a vertical edge with no slope, the query will return the lowest value of that edge.

The following query selects all values of "SysTimeSec" that are greater than or equal to 50 from the History table between 10:00 and 10:02 a.m. on December 2, 2001. No edge detection is specified.

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 = 'None'

The results are: