如何在MongoDB中使用多个键有效地执行“与众不同”？
您可以在聚合框架的帮助下使用多个键执行不同的操作。
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.distinctWithMultipleKeysDemo.insertOne({"StudentName":"Mike","StudentAge":22,"StudentMathMarks":56});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c7f74488d10a061296a3c53")
}
> db.distinctWithMultipleKeysDemo.insertOne({"StudentName":"Mike","StudentAge":22,"StudentMathMarks":56});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c7f744b8d10a061296a3c54")
}
> db.distinctWithMultipleKeysDemo.insertOne({"StudentName":"Bob","StudentAge":23,"StudentMathMarks":45});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c7f74598d10a061296a3c55")
}
> db.distinctWithMultipleKeysDemo.insertOne({"StudentName":"Bob","StudentAge":23,"StudentMathMarks":45});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c7f745e8d10a061296a3c56")
}
> db.distinctWithMultipleKeysDemo.insertOne({"StudentName":"Carol","StudentAge":27,"StudentMathMarks":54});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c7f74688d10a061296a3c57")
}在find()method的帮助下显示集合中的所有文档。查询如下-
> db.distinctWithMultipleKeysDemo.find().pretty();
以下是输出-
{
"_id" : ObjectId("5c7f74488d10a061296a3c53"),
"StudentName" : "Mike",
"StudentAge" : 22,
"StudentMathMarks" : 56
}
{
"_id" : ObjectId("5c7f744b8d10a061296a3c54"),
"StudentName" : "Mike",
"StudentAge" : 22,
"StudentMathMarks" : 56
}
{
"_id" : ObjectId("5c7f74598d10a061296a3c55"),
"StudentName" : "Bob",
"StudentAge" : 23,
"StudentMathMarks" : 45
}
{
"_id" : ObjectId("5c7f745e8d10a061296a3c56"),
"StudentName" : "Bob",
"StudentAge" : 23,
"StudentMathMarks" : 45
}
{
"_id" : ObjectId("5c7f74688d10a061296a3c57"),
"StudentName" : "Carol",
"StudentAge" : 27,
"StudentMathMarks" : 54
}这是使用多个键执行不同查询的查询-
> c = db.distinctWithMultipleKeysDemo;
test.distinctWithMultipleKeysDemo
> myResult = c.aggregate( [ {"$group": { "_id": { StudentName:"$StudentName", StudentAge: "$StudentAge" } } } ] );以下是输出-
{ "_id" : { "StudentName" : "Carol", "StudentAge" : 27 } }
{ "_id" : { "StudentName" : "Bob", "StudentAge" : 23 } }
{ "_id" : { "StudentName" : "Mike", "StudentAge" : 22 } }