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法律研究如何使用 MongoDB 選取每個組中的最大值？
可以使用 $group。讓我們建立一個包含文件的集合 −
> db.demo659.insertOne({Name:"Chris",CountryName:"US","Marks":50});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea1a50724113ea5458c7cf9")
}
> db.demo659.insertOne({Name:"David",CountryName:"US","Marks":60});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea1a50724113ea5458c7cfa")
}
> db.demo659.insertOne({Name:"Mike",CountryName:"US","Marks":55});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea1a50724113ea5458c7cfb")
}
> db.demo659.insertOne({Name:"Chris",CountryName:"UK","Marks":75});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea1a50724113ea5458c7cfc")
}
> db.demo659.insertOne({Name:"David",CountryName:"UK","Marks":54});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea1a50724113ea5458c7cfd")
}
> db.demo659.insertOne({Name:"Mike",CountryName:"UK","Marks":72});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea1a50824113ea5458c7cfe")
}藉助 find() 方法來顯示集合中的所有文件 −
> db.demo659.find();
這將生成以下輸出。它查詢每個文件（即 CountryName US 和 UK）中的最高分 −
{ "_id" : ObjectId("5ea1a50724113ea5458c7cf9"), "Name" : "Chris", "CountryName" : "US", "Marks" : 50 }
{ "_id" : ObjectId("5ea1a50724113ea5458c7cfa"), "Name" : "David", "CountryName" : "US", "Marks" : 60 }
{ "_id" : ObjectId("5ea1a50724113ea5458c7cfb"), "Name" : "Mike", "CountryName" : "US", "Marks" : 55 }
{ "_id" : ObjectId("5ea1a50724113ea5458c7cfc"), "Name" : "Chris", "CountryName" : "UK", "Marks" : 75 }
{ "_id" : ObjectId("5ea1a50724113ea5458c7cfd"), "Name" : "David", "CountryName" : "UK", "Marks" : 54 }
{ "_id" : ObjectId("5ea1a50824113ea5458c7cfe"), "Name" : "Mike", "CountryName" : "UK", "Marks" : 72 }以下是查詢以選擇每個組中的最大值 −
> db.demo659.aggregate( { $group: {
... _id: {CountryName: "$CountryName" },
... 'MaxMarks': { $max : "$Marks" }
... }})這將生成以下輸出 −
{ "_id" : { "CountryName" : "UK" }, "MaxMarks" : 75 }
{ "_id" : { "CountryName" : "US" }, "MaxMarks" : 60 }
更新於： 13-May-2020
350 次瀏覽
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