在MongoDB中使用$ match进行聚合查询是否有效？
是的，它可以在MongoDB中工作。让我们创建一个包含文档的集合-
> db.demo358.insertOne({"ClientId":101,"ClientName":"Chris","ClientAge":34});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e5692c0f8647eb59e5620cb")
}
> db.demo358.insertOne({"ClientId":102,"ClientName":"David","ClientAge":32});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e5692caf8647eb59e5620cc")
}
> db.demo358.insertOne({"ClientId":103,"ClientName":"David","ClientAge":35});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e5692d2f8647eb59e5620cd")
}
> db.demo358.insertOne({"ClientId":104,"ClientName":"Bob","ClientAge":31});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e5692ddf8647eb59e5620ce")
}
> db.demo358.insertOne({"ClientId":105,"ClientName":"Chris","ClientAge":36});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e5692e7f8647eb59e5620cf")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo358.find();
这将产生以下输出-
{ "_id" : ObjectId("5e5692c0f8647eb59e5620cb"), "ClientId" : 101, "ClientName" : "Chris", "ClientAge" : 34 }
{ "_id" : ObjectId("5e5692caf8647eb59e5620cc"), "ClientId" : 102, "ClientName" : "David", "ClientAge" : 32 }
{ "_id" : ObjectId("5e5692d2f8647eb59e5620cd"), "ClientId" : 103, "ClientName" : "David", "ClientAge" : 35 }
{ "_id" : ObjectId("5e5692ddf8647eb59e5620ce"), "ClientId" : 104, "ClientName" : "Bob", "ClientAge" : 31 }
{ "_id" : ObjectId("5e5692e7f8647eb59e5620cf"), "ClientId" : 105, "ClientName" : "Chris", "ClientAge" : 36 }这是如何与$match一起使用-
> db.demo358.aggregate(
... [ { $match : { "ClientName" : "David" }},
... { "$count": "ClientName" }
... ]
... );这将产生以下输出-
{ "ClientName" : 2 }