获取集合中每个组包含最大属性的 MongoDB 文档?
mongodbdatabasebig data analytics更新于 2026/1/19 21:37:17
您可以使用 $sort 运算符和 $group 语句获取集合中每个组包含最大属性的文档。
为了进一步理解这个概念，让我们创建一个包含文档的集合。创建包含文档的集合的查询如下: −
> db.maxAttributePerGroup.insertOne({"StudentFirstName":"John","StudentLastName":"Smith
","StudentAge":29,"StudentId":10});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c76ee341e9c5dd6f1f78277")
}
> db.maxAttributePerGroup.insertOne({"StudentFirstName":"Carol","StudentLastName":"Taylo
r","StudentAge":19,"StudentId":10});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c76ee4e1e9c5dd6f1f78278")
}
> db.maxAttributePerGroup.insertOne({"StudentFirstName":"Adam","StudentLastName":"Smit
h","StudentAge":34,"StudentId":20});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c76ee631e9c5dd6f1f78279")
}
> db.maxAttributePerGroup.insertOne({"StudentFirstName":"Bob","StudentLastName":"Taylor"
,"StudentAge":58,"StudentId":20});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c76ee791e9c5dd6f1f7827a")
}
使用 find() 方法显示集合中的所有文档。查询如下 −
> db.maxAttributePerGroup.find().pretty();
输出
{
"_id" : ObjectId("5c76ee341e9c5dd6f1f78277"),
"StudentFirstName" : "John",
"StudentLastName" : "Smith",
"StudentAge" : 29,
"StudentId" : 10
}
{
"_id" : ObjectId("5c76ee4e1e9c5dd6f1f78278"),
"StudentFirstName" : "Carol",
"StudentLastName" : "Taylor",
"StudentAge" : 19,
"StudentId" : 10
}
{
"_id" : ObjectId("5c76ee631e9c5dd6f1f78279"),
"StudentFirstName" : "Adam",
"StudentLastName" : "Smith",
"StudentAge" : 34,
"StudentId" : 20
}
{
"_id" : ObjectId("5c76ee791e9c5dd6f1f7827a"),
"StudentFirstName" : "Bob",
"StudentLastName" : "Taylor",
"StudentAge" : 58,
"StudentId" : 20
}
以下查询用于获取集合中每个组具有最大属性的文档 −
> db.maxAttributePerGroup.aggregate([
... {"$sort":{"StudentId":1,"StudentAge":-1}},
... {$group:{
... "_id":"$StudentId",
... "StudentAge":{"$first":"$StudentAge"},
... "StudentFirstName":{"$first":"$StudentFirstName"},
... "StudentLastName":{"$first":"$StudentLastName"}
... }} ]).pretty();
输出
{
"_id" : 20,
"StudentAge" : 58,
"StudentFirstName" : "Bob",
"StudentLastName" : "Taylor"
}
{
"_id" : 10,
"StudentAge" : 29,
"StudentFirstName" : "John",
"StudentLastName" : "Smith"
}
