var a = "100000";
var con = mysql.createConnection({
                    host: "xyz.com",
                    user: "db",
                    password: "db",
                    database: "truck"
         });
for (var i = 0; i < a.length; i++)
{
    var p = a.substring(0,1);//p is 1 now
    con.connect(function(err) {
    con.query("SELECT * FROM at_device_fault", function (err, result, fields) {
         con.query("SELECT * FROM at_device_fault", function (err, result, fields) {
              console.log(p);//here p is undefined here what should do for correct value(1)?
         });
    });
}