摘自：《C++ Gui Qt4编程》

一、连接数据库

bool connDB()
{
    QSqlDatabase db = QSqlDatabase::addDatabase(dbDriver);//添加驱动
    db.setHostName(hostName); //设置主机名
    db.setDatabaseName(dbName); //设置数据库名
    db.setUserName(userName); //设置用户名
    db.setPassword(userPwd); //设置用户密码

    //发送连接
    if(!db.open()) 
    {
        qDebug << db.lastError();
        return false;
    }
    return true;
}

 二、操作数据库

bool queryDB(const QString &sql)
{
    QSqlQuery query;
    query.exec(sql);
    
    if(query.next())
    {
        ...
        return true;
    }
    return false;
}

三、实例

/*
 *功能：一个登录小模块
 *软件环境：ubuntu 9.04, qt4.5, PostgreSQL8.3
 *main.cpp
*/

#include <QApplication>
#include <QtCore/QTextCodec>
#include "login.h"

int main(int argc, char *argv[])
{
	QApplication app(argc, argv);
    QTextCodec::setCodecForTr(QTextCodec::codecForName("utf-8"));

	Login *login = new Login;
	login->show();

	return app.exec();
}

/*
 *login.h
 *说明：登录主界面的头文件
*/

#ifndef LOGIN_H
#define LOGIN_H

#include <QDialog>
class QLineEdit;
class QPushButton;

class Login :public QDialog
{
	Q_OBJECT

public:
	Login(QWidget *parent = 0);

public slots:
	void logined();

private:
	QLineEdit *userNameLine;
	QLineEdit *userPwdLine;

	QPushButton *loginButton;
};
#endif

/*
 *login.cpp
 *说明：登录主界面
*/
#include <QtGui>
#include "login.h"
#include "mysql.h"

Login::Login(QWidget *parent)
	:QDialog(parent)
{
	QLabel *userNameLabel = new QLabel(tr("用户名："));
	userNameLine = new QLineEdit;

	QLabel *userPwdLabel = new QLabel(tr("密码："));
	userPwdLine = new QLineEdit;

	loginButton = new QPushButton(tr("登录"));
	connect(loginButton, SIGNAL(clicked()), this, SLOT(logined()));

	QHBoxLayout *buttonLayout = new QHBoxLayout;
	buttonLayout->addStretch();
	buttonLayout->addWidget(loginButton);

	QGridLayout *mainLayout = new QGridLayout;
	mainLayout->addWidget(userNameLabel, 0, 0);
	mainLayout->addWidget(userNameLine, 0, 1);
	mainLayout->addWidget(userPwdLabel, 1, 0);
	mainLayout->addWidget(userPwdLine, 1, 1);
	mainLayout->addLayout(buttonLayout, 2, 1);
    //mainLayout->addWidget(loginButton, 2, 1);
	setLayout(mainLayout);
	setWindowTitle(tr("登录"));
}

void Login::logined()
{
	QString name = userNameLine->text();
	QString passwd = userPwdLine->text();
    QString sql = "select name, password from users where name = '" 
		+ name + "'and password ='" + passwd + "'";

	MySql mySql;
	if( mySql.queryDB(sql) )
	{
		QMessageBox::information(this, tr("登录成功"),
				tr("登录成功！欢迎进入本系统！"),
				QMessageBox::Ok);
	}
	else
	{
		QMessageBox::warning(this, tr("登录失败"),
				tr("用户名或密码错误！是否重新登录？"),
				QMessageBox::Yes|QMessageBox::No);

	}

}

/*
 *mysql.h
 *说明：封装的SQL连接
*/

#ifndef MYSQL_H
#define MYSQL_H

#include <QObject>

class MySql :public QObject
{
public:
	MySql(QObject *parent = 0);

	bool connDB();
	bool queryDB(const QString &sql);

private:
	QString dbDriver;
	QString dbName;
	QString userName;
	QString userPwd;
	QString hostName;
	int hostPort;
};
#endif

/*
 *mysql.cpp
 *说明：封装的SQL连接
*/
#include <QtSql>
#include <QSqlDatabase>
#include <QDebug>
#include "mysql.h"

MySql::MySql(QObject *parent)
	:QObject(parent)
{
	dbDriver="QPSQL";   
	dbName="mydb";
	userName="konglingchun";
	userPwd="klcstudy";
	hostName="localhost";
	hostPort=5432;
	connDB();
}

bool MySql::connDB()
{
    QSqlDatabase db = QSqlDatabase::addDatabase(dbDriver);//添加驱动
    db.setHostName(hostName); //设置主机名
    db.setDatabaseName(dbName); //设置数据库名
    db.setUserName(userName); //设置用户名
    db.setPassword(userPwd); //设置用户密码

    //发送连接
    if(!db.open()) 
    {
	//	qDebug() << QSqlDatabase::drivers();
        qDebug() << db.lastError();
        return false;
    }
    return true;
}

bool MySql::queryDB(const QString &sql)
{
    QSqlQuery query;
    query.exec(sql);
    
    if(query.next())
    {
        qDebug() << query.value(0).toString();
        return true;
    }
    return false;
}

/*
 *mysql.sql
 *说明：在PostgreSQL数据库中建表用的sql语句
*/
-- DROP TABLE users;

CREATE TABLE users
(
  "name" character varying(20) NOT NULL,
  "password" character varying(20) NOT NULL
)
WITH (OIDS=FALSE);
ALTER TABLE users OWNER TO konglingchun;

insert into users (name, password)
    values ('001', '1234');

 注意：1、在.pro文件中，别忘了加上 QT += sql

          2、如果提示没有PostgrestSQL数据库的驱动，可以通过命令sudo apt-get install libqt4-sql-psql来安装驱动；

               如果您用的是MySQL数据库，那么命令是sudo apt-get install libqt4-sql-mysql。

              我这里有一份很全的QT4的源码包文件，想用哪个库文件直接安装就可以了。