how to implement mysql query first in nodejs? ( or to implement condition query first )

Question

it is first time to use node js So this question can be very basic question. I tried to use callback instead of just returning value but I couldn't solve the problem.

I heard that mysql query is executed after other code in node js. But I want to put (if condition) query in the middle of the code and check the condition before implementing codes below query.

I am doing IoT thing ( sensor + android app + aws ubuntu server with nodejs and using mysql for database ) I want to check if the sensor is working fine before adding data to server database. So, I put the code to check if sensors are working fine before storing data in database. ( check by comparing the results from two sensors. )

But the problem is that mysql query(to check if the sensors are right) is executed later than other code in node.js so I cannot check the condition in the middle. How can I run condition query in the middle of the code and return any condition value?



app.get('/log', function(req, res){
        r = req.query;

        result_error = error_check(r.nickname);
        console.log("error check result"+result_error);


        insert_temp(r.nickname,r.left_num,r.right_num);
        //this function is called after error_check function, but when I saw the result, console.log in this function is called earlier than error_check function 

        res.end('OK:' + JSON.stringify(req.query));
});


function error_check(nickname){
        var sql = mysql.format('select sum(left_num) as left_sum, sum(right_num) as right_sum from temp where nickname = ?  and today_date = date(current_date());',nickname);
        var query = connection.query(sql,function(err,result){
                if (err) throw err;
                console.log("Error check/// left_sum is "+result[0].left_sum + " right_sum is "+result[0].right_sum);
                if ((result[0].left_sum - result[0].right_sum >=30 )|| (result[0].right_sum - result[0].left_sum >=30 )){
                        console.log("Sensor is broken");
                        return 1;
                }
                else{
                        return -1;
                }
        });
        console.log(query);
}
/////////////////////////////but the result is
error check result undefined

(this is from insert_temp function! )nickname is ? left_num is ? right_num is ? gucheol 0 1

(this is from error_check function! this function is called before inset_temp function. error_check function is executed late...so I cannot check if sensor is wrong  )
Error check/// left_sum is 0 right_sum is 45
Sensor is broken

///but if we see the result from console/////

error check result undefined

(this is from insert_temp function! )nickname is ? left_num is ? right_num is ? gucheol 0 1

Error check/// left_sum is 0 right_sum is 45 Sensor is broken

(this is from error_check function! this function is called before insert_temp function. error_check function is executed late...so I cannot check if sensor is wrong )

You have to pass `insert_temp(r.nickname,r.left_num,r.right_num);` as a callback function to `error_check` instead of calling it immediately. Or use a promise to do the same thing. — Shilly, Jun 07 '19 at 10:03
https://stackoverflow.com/questions/14220321/how-do-i-return-the-response-from-an-asynchronous-call/14220323 technically is a duplicate, but it's so much clientside ajax and so little node.js code that it might only confuse OP more if I flag it as a dupe. — Shilly, Jun 07 '19 at 10:06

score 0 · Answer 1 · answered Jun 07 '19 at 10:43

connection.query is a asynchronous function so that will execute in end of event loop. Instant of asynchronous way you can use synchronous way using util.promisify().

You can use like below to work your code,

app.get('/log', function(req, res){
        r = req.query;

        var sql = mysql.format('select sum(left_num) as left_sum, sum(right_num) as right_sum from temp where nickname = ?  and today_date = date(current_date());',r.nickname);
        var query = connection.query(sql,function(err,result){
                if (err) throw err;
                if ((result[0].left_sum - result[0].right_sum >=30 )|| (result[0].right_sum - result[0].left_sum >=30 )){
                        console.log("Sensor is broken");
                        res.end("NOT OK: Sensor is broken");
                }
                else{
                        insert_temp(r.nickname,r.left_num,r.right_num);
                        res.end('OK:' + JSON.stringify(req.query));
                }
        });
});

Let me know if some error occur with this code.

how to implement mysql query first in nodejs? ( or to implement condition query first )

1 Answers1