Hướng dẫn join in javascript w3schools

Examples

const fruits = ["Banana", "Orange", "Apple", "Mango"];
let text = fruits.join[];

Try it Yourself »

Another separator:

const fruits = ["Banana", "Orange", "Apple", "Mango"];
let text = fruits.join[" and "];

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Definition and Usage

The join[] method returns an array as a string.

The join[] method does not change the original array.

Any separator can be specified. The default is comma [,].

Syntax

Parameters

Return Value

Browser Support

join[] is an ECMAScript1 [ES1] feature.

ES1 [JavaScript 1997] is fully supported in all browsers:

SQL JOIN

A JOIN clause is used to combine rows from two or more tables, based on a related column between them.

Let's look at a selection from the "Orders" table:

Then, look at a selection from the "Customers" table:

Notice that the "CustomerID" column in the "Orders" table refers to the "CustomerID" in the "Customers" table. The relationship between the two tables above is the "CustomerID" column.

Then, we can create the following SQL statement [that contains an INNER JOIN], that selects records that have matching values in both tables:

Example

SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate
FROM Orders
INNER JOIN Customers ON Orders.CustomerID=Customers.CustomerID;

Try it Yourself »

and it will produce something like this:

Different Types of SQL JOINs

Here are the different types of the JOINs in SQL:

• [INNER] JOIN: Returns records that have matching values in both tables
• LEFT [OUTER] JOIN: Returns all records from the left table, and the matched records from the right table
• RIGHT [OUTER] JOIN: Returns all records from the right table, and the matched records from the left table
• FULL [OUTER] JOIN: Returns all records when there is a match in either left or right table

Join Collections

MongoDB is not a relational database, but you can perform a left outer join by using the $lookup stage.

The $lookup stage lets you specify which collection you want to join with the current collection, and which fields that should match.

Consider you have a "orders" collection and a "products" collection:

orders

[
  { _id: 1, product_id: 154, status: 1 }
]

products

[
  { _id: 154, name: 'Chocolate Heaven' },
  { _id: 155, name: 'Tasty Lemons' },
  { _id: 156, name: 'Vanilla Dreams' }
]

Example

Join the matching "products" document[s] to the "orders" collection:

var MongoClient = require['mongodb'].MongoClient;
var url = "mongodb://127.0.0.1:27017/";

MongoClient.connect[url, function[err, db] {
  if [err] throw err;
  var dbo = db.db["mydb"];
  dbo.collection['orders'].aggregate[[
    { $lookup:
       {
         from: 'products',
         localField: 'product_id',
         foreignField: '_id',
         as: 'orderdetails'
       }
     }
    ]].toArray[function[err, res] {
    if [err] throw err;
    console.log[JSON.stringify[res]];
    db.close[];
  }];
}];

Run example »

Save the code above in a file called "demo_mongodb_join.js" and run the file:

Run "demo_mongodb_join.js"

C:\Users\Your Name>node demo_mongodb_join.js

Which will give you this result:

[
  { "_id": 1, "product_id": 154, "status": 1, "orderdetails": [
    { "_id": 154, "name": "Chocolate Heaven" } ]
  }
]

As you can see from the result above, the matching document from the products collection is included in the orders collection as an array.