我正在用 QT 编写程序，但在编写 SQL 查询 Select 时遇到问题。我有一个简单的表，其中包含以下列：ID、name_or_nickname、surname、职业。我想在查询中使用 3 个变量：
QString name = "Peter";
QString surname = "Smith";
QString occupation = "New York";
我想使用这些变量运行这个查询：
QSqlDatabase sql;
QSqlQuery query(sql);
QString execute = "SELECT COUNT(name) FROM table1 WHERE name_or_nickname='?' AND surname='?' AND occupation='?';";
query.prepare(execute);
query.bindValue(0, name);
query.bindValue(1, _surname);
query.bindValue(2, _occupation);
query.exec();
if (query.next())
{
rows= query.value(0).toInt();
return true;
}
else
{
qDebug() << query.lastError();
return false;
}
但是它不起作用并且 isActive() 函数返回 false，因此查询有问题 - 可能与括号有关。你能告诉我一个例子我应该如何处理它？先感谢您！
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我在这里发布必要的代码：
主窗口.h：
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <person.h>
#include <QMainWindow>
#include <QString>
#include <QtDebug>
#include <QtSql>
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = nullptr);
~MainWindow();
private slots:
void on_pushButton_1_clicked();
private:
Ui::MainWindow *ui;
QSqlDatabase sql;
bool open_or_not;
};
#endif // MAINWINDOW_H
主窗口.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
sql = QSqlDatabase::addDatabase("QSQLITE", "db");
sql.setDatabaseName("E:\\folder\\database.sqlite3");
sql.close(); // this was called beacuse of the problem with first query
sql.open();
QSqlQuery query(sql);
QString execute = "CREATE TABLE IF NOT EXISTS table1 (id INTEGER UNIQUE PRIMARY KEY, name_or_nickname TEXT, surname TEXT, occupation TEXT);";
query.exec(execute);
qDebug() << "isActive: CREATING TABLE" << query.isActive();
query.clear();
open_or_not = sql.open();
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::on_pushButton_1_clicked()
{
if(open_or_not)
{
Person person1(_sql, Johnny, Walker, California, this);
bool result = false;
result = person1.search_in_database();
qDebug() << result;
}
}
人.h：
#ifndef PERSON_H
#define PERSON_H
#include <QNetworkRequest>
#include <QNetworkReply>
#include <QtDebug>
#include <QDebug>
#include <QJsonDocument>
#include <QtSql>
namespace Ui {
class Person;
}
class Person : public QDialog
{
Q_OBJECT
public:
explicit Person(QSqlDatabase & sql, QString name_or_nickname, QString surname, QString occupation, QWidget *parent);
~Person();
bool search_in_table();
private:
Ui::Person *ui;
QSqlDatabase sql;
QString name_or_nickname;
QString surname;
QString occupation;
};
#endif // PERSON_H
个人.cpp：
#include "person.h"
#include "ui_person.h"
Person::Person(QSqlDatabase & sql, QString name_or_nickname, QString surname, QString occupation, QWidget *parent) :
QDialog(parent),
ui(new Ui::Person)
{
ui->setupUi(this);
this->sql = sql;
this->name_or_nickname = name_or_nickname;
this->surname = surname;
this->occupation = occupation;
}
Person::~Person()
{
delete ui;
}
bool Person::search_in_table()
{
QSqlQuery query(sql);
int rows = 0;
QString name = this->name_or_nickname;
QString _surname = this->surname;
QString _occupation = this->occupation;
QString execute = "SELECT COUNT(name) FROM table1 WHERE name_or_nickname='?' AND surname='?' AND occupation='?';";
query.prepare(execute);
query.bindValue(0, name);
query.bindValue(1, _surname);
query.bindValue(2, _occupation);
query.exec();
if (query.next())
{
rows= query.value(0).toInt();
return true;
}
else
{
qDebug() << query.lastError();
return false;
}
}
主要.cpp：
#include "mainwindow.h"
#include <QApplication>
#include <QPushButton>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}