Filter data based on other query, but that other query is connected in a complicated way.

[JoeCrozier]

I know there are lots of posts on here that are like "Filter data based on other query".  This is similar but I think warrants its own question.  My question is most similar to this unanswered comment on a different question.

I have a dashboard like this:

And it all pretty much 'works'. I.e. you can select a study protocol in that slicer (which is from the "Protocols" query, it'll then filter the table above to relevent Specimens (from "Specimens" query) from that protocol.

It can do this because the queries are linked together like this:

Problem is, there are a TON of protocols in that slicer that have no specimens at all. Lets say there are roughly 300 protocols to choose from, and probably like 30 that I want in the slicer because they actually have specimens.

If the data was all in one table, I'd just filter the specimen column to not be blank, but that said, its in a totally different table. I'd merge them, but given they share no common variables and only 'link' through 4 separate queries.... how could I do that?

[lbendlin]

I'd merge them, but given they share no common variables and only 'link' through 4 separate queries.... how could I do that?

You do that by using Table.AddColumn and custom column generator functions. 

Please provide sample data that covers your issue or question completely, in a usable format (not as a screenshot).
https://community.powerbi.com/t5/Community-Blog/How-to-provide-sample-data-in-the-Power-BI-Forum/ba-...
Please show the expected outcome based on the sample data you provided.

https://community.powerbi.com/t5/Desktop/How-to-Get-Your-Question-Answered-Quickly/m-p/1447523