Advanced source expression building techniques

Additional operators and XPath functions allow you to build advanced source expressions.

Navigate the data journal tree

You can build a source expression to move from child to parent nodes in the tree hierarchy using the notation ../.

To navigate from a UnitBatchJournal to a BatchJournal for the BatchID element, the syntax would be:

../../BatchID

Select an element by position

To select the last JournalEvent in the data journal, the XPath function syntax would be:

BatchJournal/UnitBatchJournals/UnitBatchJournal/JournalEvents/JournalEvent[last()]

Select an element by property

To included only those UnitBatchJournal objects that have a PIUnitName element that starts with TRA, the XPath function syntax would be:

BatchJournal/UnitBatchJournals/UnitBatchJournal[starts-with(PIUnitName,”TRA”)]

Apply TraceSet data in source expressions

The TraceSet element in the data journal is unique because it includes descriptive information about the data. You can obtain individual values from a TraceSet data journal object by specifying particular result definition types.

Obtain TraceSet descriptors

To obtain the PI Tag Name, PI Tag Descriptor, and PI Tag Engineering Units values from the element, the syntax would be:

• PI Tag Name

  TraceSet/Properties/Property[(@Descriptor='12')]

• PI Tag Descriptor

  TraceSet/Properties/Property[(@Descriptor='14')]

• PI Tag Engineering Units

  TraceSet/Properties/Property[(@Descriptor='17')]

Differentiate among TraceSet results

If you define multiple data sources within a result definition, your data journal might contain several TraceSet elements with the same Result element. This example will show how to differentiate between them.

To select the TraceSet by name, specifically the Result.Tag TraceSet, the syntax would be:

Results/Result/Value/TraceSet[Name=”Result.Tag”]

To select the TraceSet by position, specifically the second TraceSet element:

Results/Result/Value/TraceSet[position()=2]