Execution Context of a Query and Interpolation Strings¶

This section describes the concept of interpolation strings. They are used in Virtual DataPort to parameterize certain expressions used by the wrapper or the data source of a base view depending on the queries made to this view (see section Generating Wrappers and Data Sources).

An interpolation string is an expression that uses execution context variables, which generates a string as a result. A variable in an interpolation string is specified with the prefix @ followed by the name of the variable, provided that this name is a string of alphanumeric characters (letters and the characters # and _). Variables with a name that includes any other character can be specified by including the name between the symbols @{ and }.

Example: Suppose you have a Web server that allows accessing to certain reports from the departments of a particular company encoded into XML. The path to access the report from each department is the same, except for the name of the file that matches the name of the department. For example:

http://examplesite.com/exampleroute/reports/DPT1.xml

http://examplesite.com/exampleroute/reports/DPT2.xml …

Now suppose that you want to build a base view that retrieves these reports. To do so you must create an XML data source (see section XML Data Sources), an XML wrapper (see section XML Wrappers) and a base view that uses this wrapper. This base view (called DPT_REPORTS) contains a tuple for each department. Each tuple will have two attributes: DPT_NAME (text type) and REPORT, which will contain the report data.

When creating the data source for this base view, the problem arises that the data file to be accessed depends on the department referred to by the query. To solve this problem, you can add an interpolation variable to the ROUTE parameter of the CREATE DATASOURCE XML statement that creates the data source. E.g. in this example, we add the variable DPT_NAME to the URL of the Service, instead of putting the actual name of the file:

http://examplesite.com/exampleroute/reports/@{DPT_NAME}.xml

The wrappers and base views created over this data source will have a field DPT_NAME. Hence, queries such as the following can be executed:

SELECT REPORT
FROM DPT_REPORTS
WHERE DPT_NAME = 'DPT3'

And the Server will retrieve the XML document from this path

http://examplesite.com/exampleroute/reports/DPT3.xml

When an interpolation variable has a list of elements as a value (this happens in the cases of operators allowing for a list of values as operands), the value associated with the variable will be the linking of the single elements separated by the character +.

There are situations where you may need to replace an interpolation variable with the value provided in the query only if this value is not NULL and in that case, replace it with other text. In this scenario, use the interpolation function ExecuteIfIsNotNull.

The section Using the WHEREEXPRESSION Variable of the Administration Guide explains how to use this function. Although the examples provided use the interpolation variable @WHEREEXPRESSION, it can be used with any other variable.