Nested query in node js using mysql - mysql

Question

I am trying following code at node js using mysql but getting error "Cannot enqueue Query after invoking quit.
var mysql = require('mysql');
var connection = mysql.createConnection({
host : 'localhost',
user : 'USER',
password : 'PASS',
database : 'DB',
});
connection.connect();
var queryString = 'SELECT * FROM tbl_product';
connection.query(queryString, function(err, rows, fields) {
if (err) throw err;
for (var i in rows) {
console.log('Product Name: ', rows[i].product_name);
var emp_query = 'SELECT * FROM tbl_employer';
connection.query(queryString, function(emp_err, emp_rows, emp_fields) {
if (emp_err) throw emp_err;
for (var e in emp_rows) {
console.log('Employer Name: ', emp_rows[e].company_name);
}
});
}
});
connection.end();

Answer 1

I see two problems in your code:
You're calling connection.end() synchronously, but your queries run in a asynchronous flow. You have to call connection.end() only when you've finished the second query.
You're using a regular for loop to run assynchronous calls (you outter loop).
To accomplish what you're trying to do, you have to consider those assynchronous scenarios. You could use promises or a module like async, that provides you a lot of methods to deal with assyncronous flows, like async.each():
connection.query(queryString, function(err, rows, fields) {
if (err) throw err;
async.each(rows, function (row, callback) {
console.log('Product Name: ', row.product_name);
var emp_query = 'SELECT * FROM tbl_employer';
connection.query(queryString, function(emp_err, emp_rows, emp_fields) {
if (emp_err) callback(emp_err);
for (var e in emp_rows) {
console.log('Employer Name: ', emp_rows[e].company_name);
}
callback();
});
});
}, function (err) {
connection.end();
}
});
Now it will guarantee that connection.end() will just be called when all your queries have finished.

Answer 2

Remove connection end function
==> connection.end();

Answer 3

The problem is connection.end() triggered before your query is not finished yet. Try to put connection.end() to end of outer loop.
connection.query(queryString, function(err, rows, fields) {
if (err) throw err;
for (var i in rows) {
console.log('Product Name: ', rows[i].product_name);
var emp_query = 'SELECT * FROM tbl_employer';
connection.query(queryString, function(emp_err, emp_rows, emp_fields) {
if (emp_err) throw emp_err;
for (var e in emp_rows) {
console.log('Employer Name: ', emp_rows[e].company_name);
}
});
}
connection.end();
});
Hope it will be useful for you.