I want to write a filter that effectively does GROUPBY "label"

[Kevin Connell]

Users may use any number of labels so my query cannot know what labels are possible

[Nic Brough -Adaptavist-]

This isn't going to work the way you are thinking.

Filters find issues.  They have nothing to say about how they are then presented to the user (slight exception - the "order by" clause is used in a handful of places).

There is no group-by in the filters, because that's formatting, not results.

You will need to think about where you are reporting the results of running a filter and how that might "group by".

[Matt Doar]

You can sort by lables in the Issue Navigator page. But consider what happens when there are multiple labels on some issues.