Help with running a select statement from a button click

Hi everyone

This is my first post so wish me luck!

I am new to C++ and QT, i have tried to write a small application that will search an sqlite database. But i cannot figure out how to run a select statement againts the database from a pushButton_clicked

Any help would really get me started, thanks in advance.



mainwindow.h

Code:

---

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QMainWindow>
 
namespace Ui {
class MainWindow;
}
 
class MainWindow : public QMainWindow
{
Q_OBJECT
 
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
 
void on_pushButton_clicked();
 
private:
Ui::MainWindow *ui;
 
private slots:
 
void on_pushButton_2_clicked();
};
 
#endif // MAINWINDOW_H

---

(mainwindow.cpp)

Code:

---

void MainWindow:: on_pushButton_clicked()
{
 
 
bool DatabaseManager::getPerson(int ID, PersonData*& PERSON)
{
bool ret = false;
 
QSqlQuery query(QString("select * from PERSON where ID = %1").arg(ID));
if (query.next())
{
PERSON->ID = query.value(0).toInt();
PERSON->FIRSTNAME = query.value(1).toString();
PERSON->LASTNAME = query.value(2).toString();
PERSON->AGE = query.value(3).toInt();
ret = true;
}
 
return ret;
}
 
}

---

dal.h

Code:

---

#ifndef DAL_H
#define DAL_H
 
#include <QObject>
#include <QtSql/QSqlDatabase>
#include <QtSql/QSqlError>
#include <QtSql/QSqlQuery>
#include <QString>
 
class PersonData
{
public:
int ID;
QString FIRSTNAME;
QString LASTNAME;
int AGE;
};
 
 
class DatabaseManager : public QObject
{
public:
DatabaseManager(QObject *parent = 0);
~DatabaseManager();
 
public:
bool openDB();
bool getPerson(int ID, PersonData*& PERSON);
 
private:
QSqlDatabase db;
};
 
#endif // DAL_H

---

Reply With Quote




dal.cpp

Code:

---

#include "dal.h"
#include <QtSql/QSqlDatabase>
#include <QtSql/QSqlError>
#include <QtSql/QSqlQuery>
#include <QString>
#include <mainwindow.h>
#include <ui_mainwindow.h>
 
 
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
}
 
MainWindow::~MainWindow()
{
delete ui;
}
 
 
bool DatabaseManager:penDB()
{
// Find QSLite driver
db = QSqlDatabase::addDatabase("QSQLITE");
 
#ifdef Q_OS_LINUX
// NOTE: We have to store database file into user home folder in Linux
QString path(QDir::home().path());
path.append(QDir::separator()).append("mydb.sqlite");
path = QDir::toNativeSeparators(path);
db.setDatabaseName(path);
#else
// NOTE: File exists in the application private folder, in Symbian Qt implementation
db.setDatabaseName("mydb.sqlite");
#endif
 
// Open databasee
return db.open();
}

---

Hi,

several notes:

• What does not work? What does the error say. See QSqlQuery::lastError().
• We have [CODE] tags here to make source code better to read.
• Do not store a private member to a QSqlDatabase. Use the global (singleton) QSqlDatabase::database() in a local scope where you need it (so that is gets destroyed right afterwards).
• Even if it isn't a problem here, have a look at QSqlQuery::prepare().
• Don't use "*" and then address the fields on there index. Databases could (even if it is not likely) return the fields in a different order than you expect. So better use something like "SELECT id, name, foo, bar FROM ..." then you can be sure about the order.

that's all for now ;)

Quote:

---

Originally Posted by Lykurg

Hi,

several notes:

• What does not work? What does the error say. See QSqlQuery::lastError().
• We have [CODE] tags here to make source code better to read.
• Do not store a private member to a QSqlDatabase. Use the global (singleton) QSqlDatabase::database() in a local scope where you need it (so that is gets destroyed right afterwards).
• Even if it isn't a problem here, have a look at QSqlQuery::prepare().
• Don't use "*" and then address the fields on there index. Databases could (even if it is not likely) return the fields in a different order than you expect. So better use something like "SELECT id, name, foo, bar FROM ..." then you can be sure about the order.

that's all for now ;)

---

Hi Lykurg, thank you very much for your reply, i will try and reply best i can.

* What does not work? What does the error say. See QSqlQuery::lastError().

There are no errors at moment as the application just starts up and when i click on the pushButton it does nothing which is correct as i wanted the pushButton to connect to a sqlite database and run a sql statment and display the results in the listwidget

As for your other recommendations i will try and slowly digest so i can try and implement into the code. If in the meantime if you could recommend you advise a way (Code examples) i could achieve my goal i would really appriciate it.

The main issue i am having here is how to get text string from a lineedit widget and use as part of an sql statement to search a database.

Thanks again for your help, much appriciated.

It helps if you paste the actual code that is causing issues. What you posted won't compile (with the definition of getPerson inside the method on_pushButton_Clicked()). Assuming your code looks more like:

Code:

---

void MainWindow:: on_pushButton_clicked()
{
// empty?
}
 
bool DatabaseManager::getPerson(int ID, PersonData*& PERSON) 
{
  bool ret = false;
 
  QSqlQuery query(QString("select * from PERSON where ID = %1").arg(ID));
  if (query.next())
  {
    PERSON->ID = query.value(0).toInt();
    PERSON->FIRSTNAME = query.value(1).toString();
    PERSON->LASTNAME = query.value(2).toString();
    PERSON->AGE = query.value(3).toInt();
    ret = true;
  }
  return ret;
} // end of the DatabaseManager::getPerson() definition

---

Where and how do you call getPerson()?

Quote:

---

Originally Posted by ChrisW67

It helps if you paste the actual code that is causing issues. What you posted won't compile (with the definition of getPerson inside the method on_pushButton_Clicked()). Assuming your code looks more like:

Code:

---

void MainWindow:: on_pushButton_clicked()
{
// empty?
}
 
bool DatabaseManager::getPerson(int ID, PersonData*& PERSON) 
{
  bool ret = false;
 
  QSqlQuery query(QString("select * from PERSON where ID = %1").arg(ID));
  if (query.next())
  {
    PERSON->ID = query.value(0).toInt();
    PERSON->FIRSTNAME = query.value(1).toString();
    PERSON->LASTNAME = query.value(2).toString();
    PERSON->AGE = query.value(3).toInt();
    ret = true;
  }
  return ret;
} // end of the DatabaseManager::getPerson() definition

---

Where and how do you call getPerson()?

---

Sorry if i am not explaining the issue very well. You are right the code would not compile with the getPerson inside the puchButton_clicked.

Is there some way i can do this i.e. run that same code but from that click button?

Thanks in advance

Write the code into on_pushButton_clicked() or call the function from there, or make getPerson a slot and connect it with the buttons click signal.

Hi Lykurg

You have hit the nail on the head, could you maybe provide an example of how i could call that function from the pushButton, this would most definatley put me on the right track as this is what i am trying to do.

Appriciate your patience with me, thanks again.

Code:

---

void MainWindow:: on_pushButton_clicked()
{
  YourDatabaseManager->getPerson(/*...*/); // like you would call any other function
}

---

[CODE]

Quote:

---

Originally Posted by Lykurg

Code:

---

void MainWindow:: on_pushButton_clicked()
{
  YourDatabaseManager->getPerson(/*...*/); // like you would call any other function
}

---

---

Hi again

I have tried adding DatabaseManager->getPerson(); to pushButton clicked and i get the below error

unexpected unqualified '->' token


I suspected that would cause a problem as the autocomplete code feature in Qt creator didnt auto fill whille i was typing it.

Am i missing something obvious.

I would really, really, reaaally, really suggest that you learn a tiny bit of C++ before you start developing a real program. Your "problem" has nothing to do with Qt, it's simply that you don't know how to use C++ and it's not something we can teach you here.

Thanks for everyone you replied i have started reading alot more on C++.

Could anyone please explain to me the below.

Code:

---

bool getPerson(int ID, PersonData*& PERSON);

---

I know that * and & is used as pointers and are put after the text. But what is *&

Thanks in advance.

It's a reference to a pointer and I have never seen any practical use of it.