Connect MYSQL Database to QT app

Lazar1

I have created an app that calculates the gp for my university and i am supposed to connect it with a mysql db and when a user calculates his gp the grades whould be stored in the db. But after trying to do it i dont see any light when i press the button calculate the gp is ok but the numbers do not store in th db. Here is my code

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QtMath>
#include <QtSql>
#include <QtDebug>


MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");

    db.setHostName("localhost");

    db.setDatabaseName("vathmos");

    db.setUserName("root");

    db.setPassword("");

    db.open();

}




MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::on_pushButton_3_clicked()
{
float m,y,c,k;
QString x;
k=0;


x=ui->lineEdit->text();
c=x.toFloat();
if (c==0)
k=k;
else
{
y=c*5;
k=5;
}

  QSqlQuery ("INSERT INTO 'vathmoi' ( 'V_1') VALUES ('c' )") ;

x=ui->lineEdit_3->text();
c=x.toFloat();
if (c==0)
k=k;
else
{
y=y+c*6;
k=k+6;
}

QSqlQuery  ("INSERT INTO 'vathmoi' ( 'V_2') VALUES ('c' )") ;

x=ui->lineEdit_2->text();
c=x.toFloat();
if (c==0)
k=k;
else
{
y=y+c*5;
k=k+5;    
}


// there are 38 more line edits to insert the grade from each lesson and it ends like that


x=ui->lineEdit_46->text();
c=x.toFloat();
if (c==0)
k=k;
else
{
y=y+c*10;
k=k+10;
}

QString Qs;
m=y/k;

float mz,me;

Qs = QString::number( m, 'g', 10 );
ui->lineEdit_44->setText(Qs);

me=6.5;
mz=((me*213)-y)/(213-k);

if ((mz<me) || (mz>10))
    Qs="Ανέφυκτος Μέσος Όρος";
else
{
Qs = QString::number( mz, 'g', 10 );
}

ui->lineEdit_7->setText(Qs);

me=7.5;
mz=((me*213)-y)/(213-k);

if ((mz<me) || (mz>10))
    Qs="Ανέφυκτος Μέσος Όρος";
else
{
Qs = QString::number( mz, 'g', 10 );
}

ui->lineEdit_6->setText(Qs);

me=8.5;
mz=((me*213)-y)/(213-k);

if ((mz<me) || (mz>10))
    Qs="Ανέφυκτος Μέσος Όρος";
else
{
Qs = QString::number( mz, 'g', 10 );
}

ui->lineEdit_8->setText(Qs);
}

DO you have any idea how to solve this i would apriciate it very much.

mrjj

Hi and welcome

this looks a bit odd
QSqlQuery ("INSERT INTO 'vathmoi' ( 'V_2') VALUES ('c' )") ;
also, i see no exec()

Normally it would be like

QSqlQuery query;
query.prepare("INSERT INTO person (id, forename, surname) "
"VALUES (?, ?, ?)");
query.addBindValue(1001);
query.addBindValue("Bart");
query.addBindValue("Simpson");
query.exec(); // this runs it

Lazar1

Is this right?

QSqlQuery query;
query.prepare("INSERT INTO vathmoi (V_1) "
"VALUES (c)");
query.exec(); // this runs it

I want to incert the c variable to the db (the c variable is the grade that i student had in a lesson )

the_

@Lazar1

There are two ways to prepare a sql statement.

with named parameters would be

query.prepare("INSERT INTO person (id, forename, surname) "
"VALUES (:id, :forename, :surename)");

and then binding the values to the parameters with

query.bindValue(":id", id);
query.bindValue(":forename",forename);
query.bindValue(":surename", surename);

the way as @mrjj described with unnamed parameters

(I for myself prefer the named parameters for more insight with long query strings)

Lazar1

Thank you for your answer.

I used the one with the named parameters exactly as you mentioned but when i go and chech in php my admin in vathmoi table there is nothing written. and qt doesnt state any problem with the connection to the db.

the_

@Lazar1

Maybe some debug outputs will help

if(!db.open()) {
qDebug() << "DB Connect Error: " << db.lastError();
}

this will write the sql error when the database can not be opened.

if(!query.exec()) {
qDebug() << "SQL Statement Error" << query.lastError();
}

will print the error when the execution of the sql statement failed.

Lazar1

@The_

Thank you very much

while debuging with the code you worte i realized that there is no problem with the conection but with the sql statement. I am colsing this toping as solved.