Optional MultiValue usage in KQL

[rbanks54]

I have a workbook with a global parameter for holding selections made from an initial query table. I then want to use that selection to filter results in a later query.

I also want to default to showing all data when no selection has been made, and only filter when the multi-value parameter is populated.

I've set set the initial query to export it's selection parameters as follows:

I see results when I have made a selection, but I can't figure out how to create value KQL when the parameter has no selections, and is a null.

For example:

| where AppService has_any ({AppServices})
is rendered as
| where AppService has_any ()

and similarly something like this to try and work around the null doesn't parse either:
| extend myFilter = iff({AppServices:label} = '<unset>','',{AppServices})
because it's rendered as
| extend myFilter = iff('<unset>' = '<unset>','',)

Basically, I need a value other than :label that won't be nulll when the parameter is unset. Are there any tips for doing this? I can't see anything in the docs that helps me and I can't figure out how to create KQL that will be valid when the the parameter is null.

[gardnerjr]

when a multivalue param has no selected items, it has no rendered value (it is an empty string)

see https://github.com/microsoft/Application-Insights-Workbooks/blob/master/Documentation/Parameters/DropDown.md#special-casing-all

for some examples here, the normal case is to do something like this:

let selection = dynamic([{Selection}]);
SomeQuery 
| where array_length(selection) == 0  // handles no selection being treated as all
    or "*" in (selection)  // handles special case of all=* handled as all
    or  SomeField in (selection) // handles specific selected value matches

converting the selected value into a database array will work with no value there, it ends up being resolved as dynamic([]) which is a valid empty array.

if you're using ADX or Logs, you can do this directly , because they support let. If you're using ARG, where let isn't supported, you'd unfortunately have to possibly do that dynamic([{Selection}] multiple times inside the query text instead.