Connect to PostgreSQL From Qt Application with Qt Sql

Qt comes with Qt Sql APIs in order to perform database related operations. Qt SQL's APIs consist of mainly three parts.
- Driver Layer
- SQL API Layer
- User Interface Layer

In order to connect to a database from Qt application, related database driver needs to be configured.
After setting connection to a specific database from Qt application, SQL API helps to achieve common database operations such as open-close connection, query tables, etc.

Configuration required to run this application:

1-) PostgreSQL 9.4.4
2-) Qt Version 5.4.0
3-) Qt Creator 3.2.2


Create sql script for Person table: You can use pgAdmin tools in order to create tables on postgreSQL database.

-- Table: person

-- DROP TABLE person;

CREATE TABLE person
(
  id integer NOT NULL,
  name text NOT NULL,
  age integer NOT NULL,
  address character(50),
  CONSTRAINT person_pkey PRIMARY KEY (id)
)
WITH (
  OIDS=FALSE
);
ALTER TABLE person
  OWNER TO test;

Following sample project created by qt creator and contains following files :

1- QSqlConnection.pro
2- QSQLDbHelper.h
3- QSQLDbHelper.cpp
4- main.cpp

QSqlConnection.pro file contains project configuration :

TEMPLATE  = app
CONFIG   += console
CONFIG   -= app_bundle
QT       += core sql
QT       -= gui
SOURCES  += main.cpp \
    QSQLDbHelper.cpp

HEADERS += \
    QSQLDbHelper.h

In this example qt sql project QSQLDbHelper.h class header file contains database related functions.

#ifndef QSQLDBHELPER_H
#define QSQLDBHELPER_H

#include <QtSql>
#include <QString>
#include <QDebug>

class QSQLDbHelper
{
public:
    QSQLDbHelper(const char* driver);
    ~QSQLDbHelper();
    QSqlDatabase* connect( const QString& server,
                           const QString& databaseName,
                           const QString& userName,
                           const QString& password );
    void disConnect();

    int selectRowCountResult(QSqlQuery* query);
    bool executeInsert(QSqlQuery* query);
    bool executeUpdate(QSqlQuery* query);
    bool executeDelete(QSqlQuery* query);

private:
    QSqlDatabase* db;
};

#endif // QSQLDBHELPER_H

QSQLDbHelper.cpp file contains implementation details for the following functions :

1- Class constructor/desctructor
2- Db connect
3- Db disconnect
4- selectRowCount
5- insert operation
6- update operation
7- delete operation

#include "QSQLDbHelper.h"

QSQLDbHelper::QSQLDbHelper(const char* driver)
{
    db = new QSqlDatabase( QSqlDatabase::addDatabase(driver) );
}

QSQLDbHelper::~QSQLDbHelper()
{
    qDebug() << "Called Destructor!";
    delete db;
}

QSqlDatabase* QSQLDbHelper::connect( const QString& server,
                                     const QString& databaseName,
                                     const QString& userName,
                                     const QString& password )
{
    db->setConnectOptions();
    db->setHostName(server);
    db->setDatabaseName(databaseName);
    db->setUserName(userName);
    db->setPassword(password);

    if(db->open()) {
        return db;
    }
    else {
        return NULL;
    }
}

int QSQLDbHelper::selectRowCountResult(QSqlQuery* query)
{
    bool queryRes = query->exec();
    if (query->lastError().type() != QSqlError::NoError || !queryRes)
    {
        qDebug() << query->lastError().text();
        return -1;
    }

    int recordCount = 0;
    while (query->next())
    {
        qDebug() << "Field 1 : " << query->value(0).toString() 
                 << "Field 2 : " << query->value(1).toString();
        ++recordCount;
    }

    return recordCount;
}

bool QSQLDbHelper::executeInsert(QSqlQuery* query)
{
    db->transaction();
    bool queryRes = query->exec();
    if (query->lastError().type() != QSqlError::NoError || !queryRes)
    {
        qDebug() << query->lastError().text();
        db->rollback();
        return false;
    }
    db->commit();
    return true;
}

bool QSQLDbHelper::executeUpdate(QSqlQuery* query)
{
    db->transaction();
    bool queryRes = query->exec();
    if (query->lastError().type() != QSqlError::NoError || !queryRes)
    {
        qDebug() << query->lastError().text();
        db->rollback();
        return false;
    }
    db->commit();
    return true;
}

bool QSQLDbHelper::executeDelete(QSqlQuery* query)
{
    db->transaction();
    bool queryRes = query->exec();
    if (query->lastError().type() != QSqlError::NoError || !queryRes)
    {
        qDebug() << query->lastError().text();
        db->rollback();
        return false;
    }
    db->commit();
    return true;
}

void QSQLDbHelper::disConnect()
{
    qDebug() << "Disconnected From Database!";
    db->close();
}

In order to make database related operations from a qt application it is required to include "QtSql" header file into the related source file.

QSqlDatabase class represents a database in your application. A transaction is started and concluded by using rollback() and commit() functions of QSqlDatabase.

QSqlQuery class executes passed sql queries on postgresql database.

Main method in the main.cpp file calls implemented connect, disconnect and CRUD operation methods from QSQLDbHelper class.

#include <iostream>

#include <QDebug>
#include "QSQLDbHelper.h"

int main()
{
    qDebug() << "Compiled with Qt Version = " << QT_VERSION_STR;

    const char* driverName = "QPSQL";
    QSQLDbHelper* qSQLDbHelper = new QSQLDbHelper(driverName);
    QSqlDatabase* db = qSQLDbHelper->connect("localhost", "testdb", "test", "test");

    if(db->open()) {

        QSqlQuery* query = new QSqlQuery(*db);
        query->setForwardOnly(true);

        // Select empty person table
        QString name = "Paul";
        if( !query->prepare(QString("SELECT id, name from person where name = ? ")) )
        {
            qDebug() <<"Error = " << db->lastError().text();
            return -1;
        }
        else
            query->addBindValue(name);

        int queryResultRowCount = qSQLDbHelper->selectRowCountResult(query);
        qDebug() << "Initial Row Count = " << queryResultRowCount << "\n";

        // insert into empty person table
        QString id = "1";
        QString age = "34";
        QString address = "istanbul";
        if( !query->prepare(
        QString("INSERT INTO person( id, name, age, address) VALUES ( ?, ?, ?, ?)") ))
        {
            qDebug() <<"Error = " << db->lastError().text();
            return -1;
        }
        else
        {
            query->addBindValue(id);
            query->addBindValue(name);
            query->addBindValue(age);
            query->addBindValue(address);
        }

        bool result = qSQLDbHelper->executeInsert(query);
        if( result )
            qDebug() << "Successful insert";
        else
            qDebug() << "Insert failed";

        // Select person table with 1 matching record
        if( !query->prepare(
        QString("SELECT id, name from person where name = ? ")))
        {
            qDebug() <<"Error = " << db->lastError().text();
            return -1;
        }
        else
            query->addBindValue(name);

        queryResultRowCount = qSQLDbHelper->selectRowCountResult(query);
        qDebug() << "After Insert Row Count = " << queryResultRowCount << "\n";


        // Update person table
        name = "Paul2";
        if( !query->prepare(QString("UPDATE person set name=? where id =? ")) )
        {
            qDebug() <<"Error = " << db->lastError().text();
            return -1;
        }
        else
        {
            query->addBindValue(name);
            query->addBindValue(id);
        }

        result = qSQLDbHelper->executeUpdate(query);
        if( result )
            qDebug() << "Successful update";
        else
            qDebug() << "Update failed";

        // Select person table with 0 no matching record
        if( !query->prepare(
        QString("SELECT id, name from person where name = ?")) )
        {
            qDebug() <<"Error = " << query->lastError().text();
            return -1;
        }
        else
        {
            query->addBindValue(name);
        }
        queryResultRowCount = qSQLDbHelper->selectRowCountResult(query);
        qDebug() << "After Update Row Count = " << queryResultRowCount << "\n";


        // Delete from person table whose name is Paul2
        // name = "Paul2";
        if( !query->prepare(QString("Delete from person where name =? ")) )
        {
            qDebug() << "Error = " << db->lastError().text();
            return -1;
        }
        else
        {
            query->addBindValue(name);
        }

        result = qSQLDbHelper->executeDelete(query);
        if( result )
            qDebug() << "Successful delete";
        else
            qDebug() << "Delete failed";

        // Select person table with 0 no matching record
        if( !query->prepare(
        QString("SELECT id, name from person where name = ? ")) )
        {
            qDebug() << "Error = " << db->lastError().text();
            return -1;
        }
        else
        {
            query->addBindValue(name);
        }
        queryResultRowCount = qSQLDbHelper->selectRowCountResult(query);
        qDebug() << "After Delete Row Count = " << queryResultRowCount << "\n";

        delete query;
    }
    else {
        qDebug() << "Something went Wrong:" << db->lastError().text();
    }

    qSQLDbHelper->disConnect();
    delete qSQLDbHelper;

    return 0;
}

In main function database connection is established and related database related operations are being tested. Parametric, dynamic sql queries created by using addBindValue method of QSqlQuery.

Output of the execution of this program shows database operation results from Qt application in terminal.