如何在MongoDB中查询前N行？
要查询MongoDB中的前N行，可以使用聚合框架。让我们创建包含文档的集合
> db.topNRowsDemo.insertOne({"StudentName":"Larry","Score":78});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ca26eee6304881c5ce84b91")
}
> db.topNRowsDemo.insertOne({"StudentName":"Chris","Score":45});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ca26ef66304881c5ce84b92")
}
> db.topNRowsDemo.insertOne({"StudentName":"Mike","Score":65});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ca26efe6304881c5ce84b93")
}
> db.topNRowsDemo.insertOne({"StudentName":"Adam","Score":55});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ca26f066304881c5ce84b94")
}
> db.topNRowsDemo.insertOne({"StudentName":"John","Score":86});
{
"acknowledged" : true,
"insertedId" : ObjectId("5ca26f0f6304881c5ce84b95")
}以下是在find()方法的帮助下显示集合中所有文档的查询
> db.topNRowsDemo.find().pretty();
这将产生以下输出
{
"_id" : ObjectId("5ca26eee6304881c5ce84b91"),
"StudentName" : "Larry",
"Score" : 78
}
{
"_id" : ObjectId("5ca26ef66304881c5ce84b92"),
"StudentName" : "Chris",
"Score" : 45
}
{
"_id" : ObjectId("5ca26efe6304881c5ce84b93"),
"StudentName" : "Mike",
"Score" : 65
}
{
"_id" : ObjectId("5ca26f066304881c5ce84b94"),
"StudentName" : "Adam",
"Score" : 55
}
{
"_id" : ObjectId("5ca26f0f6304881c5ce84b95"),
"StudentName" : "John",
"Score" : 86
}这是您可以在MongoDB中的前N行查询的方法
> db.topNRowsDemo.aggregate([
... {$sort: {StudentName: 1}},
... {$limit: 5},
... {$match: {Score: {$gt: 65}}}
... ]).pretty();这将产生以下输出
{
"_id" : ObjectId("5ca26f0f6304881c5ce84b95"),
"StudentName" : "John",
"Score" : 86
}
{
"_id" : ObjectId("5ca26eee6304881c5ce84b91"),
"StudentName" : "Larry",
"Score" : 78
}