Delta retrieval - initial values

Initial values are special values that can be returned from queries that lie exactly on the query start time, even if there is not a data point that specifically matches the specified start time. If there is not a value exactly on the query start time, the last point before the start time will be returned with its DateTime set to the query start time and its Quality set to 133. If no value exists at or prior to the query start time, a NULL value will be returned at start time with QualityDetail set to 65536, OPCQuality set to 0, and Quality set to 1.

Querying the start time in exclusive form with the > operator indicates that a value should not be returned for the query start time if one does not exist. Querying the start time in inclusive form with the >= operator indicates that an initial value should be returned.

For example, the following exclusive query statement does not return an initial value for 2009-01-01 02:00:00.

DateTime > '2009-01-01 02:00:00'

However, the following inclusive query statement does return an initial value for 2009-01-01 02:00:00.

DateTime >= '2009-01-01 02:00:00'

No special final value is returned.