instruction: Site users overlap with SO. Original inspiration: http://data.stackexchange.com/stackoverflow/query/168043/accountid-comparison
response: SELECT 'Aviation SE User Count', COUNT(*) FROM "stackexchange.aviation".Users AS fu UNION ALL SELECT 'Users of both SO and AviationSE', COUNT(*) FROM "stackexchange.aviation".Users AS fu JOIN Users AS u ON fu.AccountId = u.AccountId