app.post('/select', (req, res) => {
  config.database = req.body.db;

  var sqlQuery = `SELECT ${req.body.select} FROM ${req.body.from}`;
  if (req.body.where !== '' && req.body.where !== undefined) sqlQuery += ` WHERE ${req.body.where}`;
  if (req.body.order !== '' && req.body.order !== undefined) sqlQuery += ` ORDER BY ${req.body.order}`;
  console.log(`Query: ${sqlQuery}`);

  sql.connect(config).then(pool => {
    return pool.request().query(sqlQuery)
  }).then(result => {
    sql.close();
    sqlDone = true;
    console.dir(result);
    res.header('Content-Type', 'Application/JSON');
    res.json(result.recordset);
  }).catch(err => {
    sql.close();
    sqlDone = true;
    console.log('Caught Error:');
    console.log(err);
  });
  sql.on('error', err => {
    sql.close();
    console.log('SQL Request Error:');
    console.log(err);
  });
});

Error: Global connection already exists. Call sql.close() first.