[{"values":[{"value":"a"},{"value":"b"},{"value":"c"},{"value":"a"},{"value":"b"},{"value":"c"}]},{"values":[{"value":"a"},{"value":"b"},{"value":"d"},{"value":"g"},{"value":"d"},{"value":"c"}]}]

db.collection.aggregate([{$unwind:"$values"},{$group:{_id:"$values.value",count:{$sum:1}}}])