I usually use bash or python scrips to query data from multiple wikis at
once, but I wanted something "simple" in Quarry such as a list of users
who are active in two wikis, and I felt very stupid.
I ended up using two separate queries, a subquery and a mix of
GROUP_CONCAT, TO_BASE64 and SET SESSION group_concat_max_len (to make a
sort of temporary table in a pseudo-array string):
https://quarry.wmflabs.org/query/23956
This was reasonably fast, but is there a better way?
Federico