MongoDB SELECT COUNT GROUP BY？
您可以在聚合框架的帮助下实现这一目标。为了理解这个概念，让我们用文档创建一个集合。使用文档创建集合的查询如下-
> db.countGroupByDemo.insertOne({"StudentId":10,"StudentName":"John"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c7700871e9c5dd6f1f78296")
}
> db.countGroupByDemo.insertOne({"StudentId":10,"StudentName":"Carol"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c77008f1e9c5dd6f1f78297")
}
> db.countGroupByDemo.insertOne({"StudentId":20,"StudentName":"Sam"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c7700971e9c5dd6f1f78298")
}
> db.countGroupByDemo.insertOne({"StudentId":30,"StudentName":"Mike"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c7700a21e9c5dd6f1f78299")
}
> db.countGroupByDemo.insertOne({"StudentId":30,"StudentName":"David"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c7700aa1e9c5dd6f1f7829a")
}
> db.countGroupByDemo.insertOne({"StudentId":10,"StudentName":"Maxwell"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c7700b41e9c5dd6f1f7829b")
}
> db.countGroupByDemo.insertOne({"StudentId":20,"StudentName":"Bob"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c7700bd1e9c5dd6f1f7829c")
}在find()method的帮助下显示集合中的所有文档。查询如下-
> db.countGroupByDemo.find().pretty();
输出结果
{
"_id" : ObjectId("5c7700871e9c5dd6f1f78296"),
"StudentId" : 10,
"StudentName" : "John"
}
{
"_id" : ObjectId("5c77008f1e9c5dd6f1f78297"),
"StudentId" : 10,
"StudentName" : "Carol"
}
{
"_id" : ObjectId("5c7700971e9c5dd6f1f78298"),
"StudentId" : 20,
"StudentName" : "Sam"
}
{
"_id" : ObjectId("5c7700a21e9c5dd6f1f78299"),
"StudentId" : 30,
"StudentName" : "Mike"
}
{
"_id" : ObjectId("5c7700aa1e9c5dd6f1f7829a"),
"StudentId" : 30,
"StudentName" : "David"
}
{
"_id" : ObjectId("5c7700b41e9c5dd6f1f7829b"),
"StudentId" : 10,
"StudentName" : "Maxwell"
}
{
"_id" : ObjectId("5c7700bd1e9c5dd6f1f7829c"),
"StudentId" : 20,
"StudentName" : "Bob"
}这是按以下条件选择计数组的查询：
> db.countGroupByDemo.aggregate([
... {"$group":{_id:"$StudentId",counter:{$sum:1}}}]);输出结果
{ "_id" : 30, "counter" : 2 }
{ "_id" : 20, "counter" : 2 }
{ "_id" : 10, "counter" : 3 }