MongoDB Aggregate限制记录数
要限制记录数，请在MongoDB中使用$limit。让我们创建一个包含文档的集合-
> db.demo240.insertOne({"StudentName":"Chris"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e441d969af932883c61ea3c")
}
> db.demo240.insertOne({"StudentName":"Bob"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e441d9a9af932883c61ea3d")
}
> db.demo240.insertOne({"StudentName":"David"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e441d9d9af932883c61ea3e")
}
> db.demo240.insertOne({"StudentName":"Mike"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5e441da19af932883c61ea3f")
}在find()方法的帮助下显示集合中的所有文档-
> db.demo240.find();
这将产生以下输出-
{ "_id" : ObjectId("5e441d969af932883c61ea3c"), "StudentName" : "Chris" }
{ "_id" : ObjectId("5e441d9a9af932883c61ea3d"), "StudentName" : "Bob" }
{ "_id" : ObjectId("5e441d9d9af932883c61ea3e"), "StudentName" : "David" }
{ "_id" : ObjectId("5e441da19af932883c61ea3f"), "StudentName" : "Mike" }以下是限制MongoDB中记录数的查询-
> db.demo240.aggregate( { $limit : 3 });这将产生以下输出-
{ "_id" : ObjectId("5e441d969af932883c61ea3c"), "StudentName" : "Chris" }
{ "_id" : ObjectId("5e441d9a9af932883c61ea3d"), "StudentName" : "Bob" }
{ "_id" : ObjectId("5e441d9d9af932883c61ea3e"), "StudentName" : "David" }