nodejs connected table query

Node.js is a JavaScript runtime based on the Chrome V8 engine that can run JavaScript on the server side and is very suitable for writing web applications. When developing web applications, using a SQL database is a very common approach. When using a SQL database, it is often necessary to perform connection queries to obtain relevant data.

This article will introduce how to use Node.js for connection query. We will use the relational database MySQL as an example. Here, I assume that you already know about MySQL database and how to use MySQL connection pool in Node.js.

Join query is a very basic operation in SQL, used to find related data between multiple tables. In this article, we will use the following two tables to perform join queries:

• Student table: including the student's ID, name and score
• Class table: including the class's ID and Name

We will perform a connection query through the class ID in the student table and the ID in the class table to obtain the name of each student's class.

First, we need to prepare our data table. We will create a table named "student" and a table named "class". The student table will contain three fields: id, name and class_id. The class table will contain two fields: id and name. Create the data table in the MySQL database.

The following is the SQL command to create the table.

CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) NOT NULL,
  `class_id` int(11) NOT NULL,
  `score` int(11) NOT NULL,
  PRIMARY KEY (`id`)
);
CREATE TABLE `class` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(50) NOT NULL,
  PRIMARY KEY (`id`)
);

Now that we have prepared the data table, the next step is to use Node.js to perform connection queries.

1. Import the MySQL module and MySQL connection pool:

First, install node-mysql in our project, which is a popular MySQL module that allows us to use Node .js for MySQL database connection and query.

npm install mysql --save

Then we need to import mysql and mysql connection pool in our application:

const mysql = require('mysql');
const pool = mysql.createPool({
    host: 'localhost',
    user: 'root',
    password: 'password',
    database: 'my_database'
});

2. Write the query statement:

Next, we A query statement needs to be written. The student table and class table can be connected through the JOIN keyword. We specify the join conditions between the two tables through the ON clause.

SELECT student.id, student.name, class.name as class_name, student.score FROM student 
JOIN class ON student.class_id = class.id

The above query statement will return the student's ID, name, class name and score.

3. Execute the query:

Now, we have our query code ready. We can use MySQL connection pool to execute the query and save the results in an array. In the code below, we use the connection.query() statement to execute the query.

pool.getConnection((err, connection) => {
    if (err) throw err;
    connection.query('SELECT student.id, student.name, class.name as class_name, student.score FROM student JOIN class ON student.class_id = class.id', function (error, results, fields) {
        if (error) throw error;
        console.log(results);
        connection.release();
    });
});

The above code queries the database and prints the results to the console.

The above is the entire process of using Node.js to perform connection queries. When using Node.js for database connections and queries, we need to pay attention to the following points:

1. Use connection pool: Connection pool can significantly improve the performance of the application.
2. Use prepared statements: Prepared statements can prevent SQL injection attacks and improve application security.
3. Close the connection: After using the database connection, close the connection to release resources.

In this article, we demonstrate how to use Node.js for connection queries. Join query is one of the basic operations in SQL and is very useful. By using Node.js and MySQL connection pool, we can easily perform connection queries and get the required data.

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