MySQL examples in Node.js

If you’re integrating your Node.js service with MySQL, you probably want to execute queries.

I’ve seen a fair bit of ugly code to do this, often using callbacks. I thought I would share some of my own patterns here. Maybe this is useful to others to read.

I don’t typically use ORM’s. If you’re already happy with sequalize, this article is probably not for you.

Prequisites

The most popular NPM package for MySQL is mysql, but I use the mysql2 package instead.

The mysql2 package is also very popular, but has a few advantages. The advantage I care about is support for promises. The authors of this package have actually teamed up with the authors of the mysql package to reduce double efforts, and was made to be compatible with mysql so for many people it’s a drop-in replacement.

Creating a pool

const mysql = require('mysql2/promise');
const pool = mysql.createPool({
  host: 'localhost',
  user: 'root',
  database: 'test',
  waitForConnections: true,
  connectionLimit: 10,
  queueLimit: 0
});

A pool manages multiple connections. I create a pool once per node.js server.

Note: if you are migrating from the mysql to mysql2 package, you probably don’t want to require mysql2/promise. You can instead require mysql2 to get full backwards compatibility and use the promise() functions to get access to promisified versions of the APIs.

Note2: mysql2 uses the utf8mb4 charset by default. If you don’t know what this means trust that it’s the right choice. If you do know what this is, rejoice that somebody finally picked a sane default.

Executing a SELECT query

Generally I don’t need transactions for SELECT queries, because of this I can simply ask the pool to execute the query.

async function getBlogPost(id) {

  const result = await pool.query('SELECT * from posts WHERE id = ?', [id]); 
  if (!result[0].length < 1) {
    throw new Error('Post with this id was not found');
  }
  return result[0][0];

}

result[0][0] looks a bit weird, this is because the result of the query returns a tuple (array) with 2 items. The first item has the result of the query, and the second has the meta data. This subtle decision is my #1 complaint about the library because it makes a lot of things slightly uglier than they need to be.

So if we want just the first record of the result, this would you need to use result[0][0].

Whenever I write a function that should return exactly 1 item, I will either return an item or throw an error. I don’t return undefined or null.

A SELECT query that returns multiple records is more elegant:

async function getAllBlogPost() {

  const result = await pool.query('SELECT * from posts'); 
  return result[0];

}

Whenever I write a function that returns 0 or more items, this function always returns an array with 0 or more items. If the collection is empty, I return an empty array.

Note: sometimes there is a distinction between an empty collection or a collection not existing. If that distinction exists, I do throw an error in the latter case.

Executing an INSERT query

Generally when doing INSERT queries, most people use the following syntax:

INSERT INTO posts (title, body) VALUES (?, ?)

MySQL also has a second system for inserting that is less popular, and looks more like an UPDATE statement:

INSERT INTO posts SET title = ?, body = ?

The second syntax is the one I use. A big advantage is that the ‘value’ is close to the name of the field. If you ever had to count questionmarks, you know why this is nice.

Naively you can execute this query as follows:

async function insertPost(title, body) {

  await pool.query(
    'INSERT INTO posts SET title = ?, body = ?',
    [ title, body ]
  );

}

But there is a nicer way to do this:

async function insertPost(title, body) {

  await pool.query(
    'INSERT INTO posts SET ?',
    { title, body } 
  );

}

If you are used to MySQL prepared statements, you might wonder why does this work?

The reason is that the placeholder ? gets special treatement when you pass objects or arrays to it.

Specifically, if you pass an array like this:

['foo', 'bar']

It expands to

'foo', 'bar'

And objects such as this:

{ foo: 'bar', gaz: 'zim' }

Expand to:

`foo` = 'bar', `gaz` = 'zim`

Here’s another neat example of this:

async function getByCategoryIds(ids) {

  // Pretends ids is [1, 2]
  const result = await pool.query(
    'SELECT * from posts WHERE category_id IN (?)',
    [ids],
  ); 
  return result[0];

}

This actually works. If you are well versed with how MySQL works you might scream at this point: “This doesn’t use real prepared statements”, and you would be right.

Prepared statements

Both the mysql and mysql2 package by default emulate prepared statements client-side. A lot of people feel that this is a really bad security practice.

I disagree, and might write about that in another article at one point. Ask most security experts though and they’ll tell you this is bad.

To do a real prepared statements, you have to use the execute() function:

async function insertPost(title, body) {

  await pool.execute(
    'INSERT INTO posts SET title = ?, body = ?',
    [ title, body ]
  );

}

This uses real MySQL prepared statements, but unfortunately doesn’t let you work with arrays and objects.

Running multiple queries on a single connection

Every time you call .query() or .execute(), you might get a new connection from the pool. Sometimes it’s a good idea to use the same connection if you do multiple queries.

One reason might be that you have multiple replicated MySQL servers and you want to ensure that you’re running the queries on the same machine with the same state.

To do this, you need to grab a connection from the pool, and release it once you’re done.

The ideal pattern for me looks like this:

async function batchingThings() {

  const connection = await pool.getConnection();
  try {

    await connection.query('...');
    await connection.query('...');

  } finally {

    connection.release();

  }

}

Remember that if you run multiple queries on a single connection they must be serialized. They must not be parallel.

Transactions

If you run multiple queries that change the state of the database, it’s often a very good idea to have them all succeed or fail as a group.

Transactions are used for that. After a transaction has started its possible to roll back every query since the start of the transaction.

The pattern I use for this is similar to the last, but a bit more complex:

async function batchingThings() {

  const connection = await pool.getConnection();
  await connection.beginTransaction();

  try {

    await connection.query('...');
    await connection.query('...');

    await connection.commit();

  } catch (err) {

    await connection.rollback();
    // Throw the error again so others can catch it. 
    throw err;

  } finally {

    connection.release();

  }

}

If my transaction code becomes too complex and I need to split it up over multiple functions, I pass the connection object around as the argument.

Typescript everything

I wanted my examples to be accessible, but all my code is written in Typescript. Every MySQL table row has its own type and I’m as strict as I can.

This is a really good idea. If there’s interest, I can write another post with typescript and mysql2 examples.

Questions / comments?

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  2. If you’re a dev, you can also send a pull request and edit in your comment in this article.

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