mongodb group 查询

db.getCollection("work_order").aggregate([
    {
        $group : {
            _id : "$car_brand",
            count: { $sum : 1}
        }
    }
])

db.getCollection("work_order").aggregate([
    {
        $group : {
            _id : "$car_series",
            count: { $sum : 1}
        }