MongoDB equivalent of an SQL query to get the distinct values of a field in a collection including the count of documents which have each distinct value (distinct with count)

mongodb_distinct_count.js

//equivalent of MySQL SELECT COUNT(*) AS cnt, fieldName FROM someTable GROUP BY fieldName;
db.someCollection.aggregate([{"$group" : {_id:"$fieldName", cnt:{$sum:1}}}]);

//as above but ordered by the count descending
//eg: SELECT COUNT(*) AS cnt, fieldName FROM someTable GROUP BY fieldName ORDER BY cnt DESC;
db.someCollection.aggregate([{"$group" : {_id:"$fieldName", cnt:{$sum:1}}}, {$sort:{'cnt':-1}}]);

db.order.aggregate([{ $group: { _id: "$seller_email", total_order: { "$sum": 1 },
                              seller_address_1 : { $first: '$seller_address_1' },
                              seller_country : { $first: '$seller_country' },
                              seller_id : { $first: '$seller_id' },
                              seller_phone : { $first: '$seller_phone' },
                              seller_city : { $first: '$seller_city' },
                              seller_email : { $first: '$seller_email' },
                              seller_name : { $first: '$seller_name' },
                              seller_postcode : { $first: '$seller_postcode' }}},
                    { $out: "seller_info" }
                   ], 
                   { allowDiskUse: true})