An example of a transaction

query.js

'use strict';

const {query} = require('./mysql-wrapper');

const sql = `
START TRANSACTION;
  # Calculate balance
  SELECT @balance := SUM(amount) FROM transactions WHERE accountId = 1 FOR UPDATE;
  
  # Compare balance with the amount we want to withdraw
  SET @finalAmount = IF(@balance >= 100, -100, NULL);
  
  # If our balance was too low then this will fail as NULL is not allowed as transaction amount
  INSERT INTO transactions (amount, accountId) VALUES (@finalAmount, 1);
COMMIT;
`;

query({sql})
  .then(result => {
    console.log('Success', result);
  })
  .catch(err => {
    console.error('Failed', err);
  });