Multiple dependent SQL queries in Node.js

Question

Im trying to build an app which requires that I make more than one query per view/controller.

Im trying to do the asynchronously using a nested structure, but the sql result comes up undefined in the inner closure.

This is the function Im using to do this:

var conn = db.config(mysql);

function run_queries(conn,callback) {

    conn.query(sql1, var1, function(err, res1) {

        if(err){ callback(err); return; }

        console.log(res1);     // RETURNS SUCCESSFULLY!

        conn.query(sql2, var2, function(err, res2) {

            if(err){ callback(err); return; }

            console.log(res2); // UNDEFINED :(

            callback(null, res2);

        });

    });

}

run_queries(conn,function(err, result){

    console.log(result);       // UNDEFINED :(

});

I have checked my SQL and it is without errors. If I swap sq1 with sq2, the outer closure returns the correct query for sq2.

The inner closure just isnt returning a query.

caolan.github.io/async it's very important to understand async. I am going to use async.parallel and give you example for you. — Yogesh Patel
– Yogesh Patel, Commented Nov 19, 2016 at 16:33
much appreciated. I have used async to parse post data, but have not explored the module further. your response below was very helpful. thank you! just out of curiosity and for learning purposes, why do you think my code above was returning undefined in the inner closure? — yevg
– yevg, Commented Nov 19, 2016 at 20:24
asynchronous feature of nodeJs. can you try use setTimeout(function(){ \\ your conn.query code },0). — Yogesh Patel
– Yogesh Patel, Commented Nov 20, 2016 at 13:36

Yogesh Patel · Accepted Answer · 2016-11-19 17:00:23Z

2

Async is a very useful for an asynchronous call. You can make your code well maintained using an async module.

Example of your code using async.parallel

var conn = db.config(mysql);
var async = require('async');

function run_queries(conn, cb) {


    async.parallel({

      res1: function (cb) {

        conn.query(sql1, var1, function (err, res1) {
          if (err) { cb(err) }
          else { cb(null, res1) }
        })
      },
      res2: function (cb) {

        conn.query(sql2, var2, function (err, res2) {
          if (err) { cb(err) }
          else { cb(null, res2) }
        })
      }
    },
    function (err, result) {
      if (err) { cb(err) }
      else { cb(null, result) }
    })

}

run_queries(conn, function (err, result) {
  console.log(result);       // UNDEFINED :(
  // GET OBJECT OF res1 AND res2
});

edited Nov 19, 2016 at 17:00

answered Nov 19, 2016 at 16:54

Yogesh Patel

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1 Comment

yevg Over a year ago

this worked and the structure is much simpler such that I can create a function for an arbitrary number of queries. thank you!!

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