Data Structure
Networking
RDBMS
Operating System
Java
MS Excel
iOS
HTML
CSS
Android
Python
C Programming
C++
C#
MongoDB
MySQL
Javascript
PHP
Physics
Chemistry
Biology
Mathematics
English
Economics
Psychology
Social Studies
Fashion Studies
Legal Studies如何从 MongoDB 数组中获取特定元素？
你可以使用 aggregate 框架从 MongoDB 数组中获取特定元素。为了理解这个概念，让我们创建一个包含文档的集合。创建包含文档的集合的查询如下 –
> db.getParticularElement.insertOne({"InstructorName":"Larry","InstructorTechnicalSubject":["Java","C","C++","MongoDB","MySQL","SQL Server"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5c7ee027559dd2396bcfbfb1")
}使用 find() 方法显示集合中的所有文档。查询如下 –
> db.getParticularElement.find().pretty();
以下是输出 –
{
"_id" : ObjectId("5c7ee027559dd2396bcfbfb1"),
"InstructorName" : "Larry",
"InstructorTechnicalSubject" : [
"Java",
"C",
"C++",
"MongoDB",
"MySQL",
"SQL Server"
]
}以下是获取数组中特定元素的查询 –
> db.getParticularElement.aggregate([
... {
... $project:
... {
... ElementFromAnArray: 1,
... FourthElement: { $arrayElemAt: [ "$InstructorTechnicalSubject", 3] },
...
... }
... }
... ]);以下是输出 –
{ "_id" : ObjectId("5c7ee027559dd2396bcfbfb1"), "FourthElement" : "MongoDB" }
更新于： 2019 年 7 月 30 日
149 次浏览
广告