Table merge transform but "inner" join

With a Prometheus backend, I am trying to filter on the results of a few queries after running the Merge transform on them. To visualise:

Query a (after filtering on a label):
a | 1
b | 2

Query b (which doesn’t have the filter from a):
a | 1
b | 2
c | 3

Merge gives the following table:

a | 1 | 1 
b | 2 | 2
c |   | 3

But I don’t want to see row c. How can I remove it?

I can do a join with metric a in each of the subsequent Prometheus queries, but I’d rather be able to to it in Grafana as there are a number of those queries.

This is technically not possible at the moment.

Thanks for the confirmation. In the meantime the Prometheus queries would look something like:

metric_a{filter_label="value"} * on (id_label) group_left() metric_b