Aggregation-聚合查询

一、Aggregation-聚合查询和mysql sql语句对应

Aggregation：

参数说明：sql(Operators)

     where ($match) 、group by ($group) 、having($match)、select($project)、order by($sort)、limit($limit)

     sum($sum)、count($sum)、join($lookup)

SELECT cust_id, SUM(price) as total
FROM orders
WHERE status = 'A'
GROUP BY cust_id
HAVING total > 250

db.orders.aggregate([
    {$match: {status: 'A'}},
    {$group: {_id: "$cust_id",total: { $sum: "$price"}}},
    {$match: {total: { $gt: 250}}}
])

二、更加字段长度排序

db.collection.aggregate(
    [
        {$project: {
            "field": 1,
            "field_length": { $strLenCP: "$field" }
        }},
        {$sort: {"field_length": -1}},
        {$project: {"field_length": 0}},
    ]
)