as follows:
after Filter, you are grouped. The two fields are grouping conditions.
however, I just want to get the time in the packet, that is, the data whose fdate is the largest.
how can I do that?
db.SEC_2018_05_12.aggregate([{ $match : { fyear : { $eq : 2018},fmonth:{$eq : 5},fday:{$eq : 12},fhour:{$eq : 17},fmin:{$eq : 8} } }
, { $group: { _id: {dev_id: "$dev_id", data_id: "$data_id"},maxTimeValue: { $max: "$fdate" } } } ]).pretty() if you can give some test conditions, it will be easier for others to understand your problem.
first of all, the operator $eq, is not necessary to write most of the time. So your query is equivalent to:
{
fyear: 1,
fmonth: 1,
fday: 1,
fhour: 1,
fmin: 1,
dev_id: 1,
data_id: 1,
fdate: -1
}looks scary enough, doesn't it? But in fact, the previous pile of time itself is just a time.
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