const express = require('express');
const app = express();
const mysql = require('mysql');
...
const db = mysql.createConnection({
	...
});
db.connect((err) => {
	...
});
...
app.get('/foo', function(req, res){
	...
	let sql0 = "select ... from ... where ...";
	let qry0 = db.query(sql0, function(err0, result0){
		...
		result0.forEach(function(row0) {
			...
			let sql1 = "select ... from ... where ...";
			let qry1 = db.query(sql1, function(err1, result1){
				...
				result1.forEach(function(row1) {
					...
					let sql2 = "select ... from ... where ...";
					let qry2 = db.query(sql2, function(err2, result2){
						...
						result2.forEach(function(row2) {
							...
							let sql3 = "select ... from ... where ...";
							let qry3 = db.query(sql3, function(err3, result3){
								...
								result3.forEach(function(row3) {
									...
									
								});
							});
						});
					});
				});
			});
		});
	});
});
...
app.listen(app.get('port'), function(){
	console.log("Express started");
});