Filtered Data Queries
You can retrieve data using an optional filter tag or filter expression if the client program or API you are using supports it.
Normally, a data query specifies a start and an end time for the query. Data is returned for ALL intervals between the start and end times. A filtered data query allows you to specify a filter tag or expression with additional criteria so that only some of those intervals which match the filter conditions are returned. Themethod of calculating the value attributed to the interval can be different from a non-filtered query, since the filter criteria can exclude raw samples inside an interval as well as exclude intervals themselves.
The value that triggers a transition from FALSE to TRUE can be a raw value or interpolated value. If a FilterTag or expression is supplied, the Data Archiver attempts to filter time periods from the results.
The filter data query parameters include:
- FilterTag or FilterExpression
- FilterMode
- FilterComparisonMode
- FilterComparisonValue
Each parameter is described in the following table with examples that demonstrate common usages.
Internally to the Data Archiver, the filter condition is evaluated to get zero or more time ranges. For example, if you query from 1pm to 2pm and the filter condition was never TRUE during that time, nothing is returned.
If the condition was TRUE from 1:40 to 1:45 then only the data for that time range is queried and returned. Together the Filter Tag, Filter value, and Filter Comparison Mode define the criteria to apply to each interval to determine inclusion or exclusion. You can optionally use Filter Expression to include all the above parameters in one condition.
The Include Times defines how the time periods before and after transitions in the filter condition should be handled. An example with actual data and a graphic to clarify the behavior of each of the IncludeTime options is provided in the following topics.
You can retrieve data Using a Filtered Data Query and Using Filter Expressions.