MongoDB组查询以获取文档中重复标记的数量？
对于组查询，请使用MongoDB$group并使用$sum获取计数。让我们创建一个包含文档的集合-
> db.demo676.insertOne({"Marks":87});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea41eed04263e90dac943f2")
}
> db.demo676.insertOne({"Marks":75});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea41ef304263e90dac943f3")
}
> db.demo676.insertOne({"Marks":87});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea41ef404263e90dac943f4")
}
> db.demo676.insertOne({"Marks":65});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea41ef704263e90dac943f5")
}
> db.demo676.insertOne({"Marks":65});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ea41ef804263e90dac943f6")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo676.find();
这将产生以下输出-
{ "_id" : ObjectId("5ea41eed04263e90dac943f2"), "Marks" : 87 }
{ "_id" : ObjectId("5ea41ef304263e90dac943f3"), "Marks" : 75 }
{ "_id" : ObjectId("5ea41ef404263e90dac943f4"), "Marks" : 87 }
{ "_id" : ObjectId("5ea41ef704263e90dac943f5"), "Marks" : 65 }
{ "_id" : ObjectId("5ea41ef804263e90dac943f6"), "Marks" : 65 }以下是获取重复标记计数的查询-
> db.demo676.aggregate( { $group: {
... _id: {Marks: "$Marks" },
... 'Count': { $sum :1 }
... }})这将产生以下输出-
{ "_id" : { "Marks" : 75 }, "Count" : 1 }
{ "_id" : { "Marks" : 65 }, "Count" : 2 }
{ "_id" : { "Marks" : 87 }, "Count" : 2 }