使用 mongodb 中的聚合获取所有具有最大值的文档 [英] get all the documents having max value using aggregation in mongodb
问题描述
I want to fetch "all the documents" having highest value for specific field and than group by another field.
Consider below data:
_id:1, country:india, quantity:12, name:xyz
_id:2, country:USA, quantity:5, name:abc
_id:3, country:USA, quantity:6, name:xyz
_id:4, country:india, quantity:8, name:def
_id:5, country:USA, quantity:10, name:jkl
_id:6, country:india, quantity:12, name:jkl
Answer should be
country:india max-quantity:12
name xyz
name jkl
country:USA max-quantity:10
name jkl
I have tried several queries, but I can get only the max value without the name or i can go group by but it shows all the values.
db.coll.aggregate([{
$group:{
_id:"$country",
"maxQuantity":{$max:"$quantity"}
}
}])
for example above will give max quantity on every country but how to combine with other field such that it shows all the documents of max quantity.
If you want to keep document information, then you basically need to $push
it into an array. But of course, then having your $max
values, you need to filter the contents of the array for just the elements that match:
db.coll.aggregate([
{ "$group":{
"_id": "$country",
"maxQuantity": { "$max": "$quantity" },
"docs": { "$push": {
"_id": "$_id",
"name": "$name",
"quantity": "$quantity"
}}
}},
{ "$project": {
"maxQuantity": 1,
"docs": {
"$setDifference": [
{ "$map": {
"input": "$docs",
"as": "doc",
"in": {
"$cond": [
{ "$eq": [ "$maxQuantity", "$$doc.quantity" ] },
"$$doc",
false
]
}
}},
[false]
]
}
}}
])
So you store everything in an array and then test each array member to see if it's value matches the one that was recorded as the maximum, discarding any that do not.
I'd keep the _id
values in the array documents since that is what makes them "unique" and won't be adversely affected by $setDifference
when filtering out values. But of course if "name" is always unique then it won't be required.
You can also just return whatever fields you want from $map
, but I'm just returning the whole document for example.
Keep in mind that this has the limitation of not exceeding the BSON size limit of 16MB, so is okay for small data samples, but anything producing a potentially large list ( since you cannot pre-filter array content ) would be better of processed with a separate query to find the "max" values, and another to fetch the matching documents.
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