Database
[{subs:[{subsub:null},{subsub:{a:"a",b:"b"}},{subsub:{a:"c",b:"d"}}]},{subs:[{subsub:null},{subsub:{a:"e",b:"f"}},{subsub:{a:"g",b:"h"}}]},]
Query
db.collection.aggregate([{$unwind:"$subs"},{$match:{"subs.subsub":{$ne:null}}},{$group:{_id:null,data:{$addToSet:{a:"$subs.subsub.a"}}}},{$unwind:"$data"},{$replaceRoot:{newRoot:"$data"}}])