Question

我有一个方法可以连接到SQLITE并按条件使用数据库（在我的数据库中插入数据或创建表。）

当我在我的代码中调用它时，首先正确打开数据库但无法创建表。意味着查询不显示错误但不执行。

这是我的方法：

void SQL(string cmnd)
{

    QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");//not dbConnection
    QString path = "DBAbsent";
    db.setDatabaseName(path);
    QSqlQuery q(db);

    if(!db.open())
    {

        printf("Error to open database");
    }
    else
    {

        if (cmnd=="createTable")
        {
            q.prepare("create table IF NOT EXISTS Personel (P_id int primary key, "
                                 "CardID varchar(50), "
                               "name varchar(50));");

            if(!q.exec())
        {
            qDebug()<<strerror(errno);
           qDebug()<<q.lastError(); // show this error : QSqlError("", "Unable to fetch row", "No query")

        }

            q.prepare("create table IF NOT EXISTS Absent "
                                 "(P_id int , "
                                 "DateTime varchar(50), "
                                 "key1 int,key2 int,key3 int,key4 int);");
            if(!q.exec())
                qDebug()<<strerror(errno);

             db.close();
             return;

        }
        else if (cmnd=="Register")
        {
            string card=ReadFromCard();
            printf("\nCard Accepted : %s\nEnter Your name: ",card.c_str());
            string name;
            int id;
            //string dt=QDateTime::currentDateTime().toString("yyyy-MM-dd HH:mm:ss");
            cin>>name;
            printf("\n Name Accepted.\nEnter Your Personel ID: ");
            cin>>id;
            q.prepare( "SELECT count(P_id) FROM Personel WHERE P_id =?;" );
            q.bindValue(0,id);

            if( !q.exec() )
            {
              printf("\nThe error occured from P_ID database");
              qDebug()<<strerror(errno);
              //ShowMainMenu();
              //return;
            }

            // Note: if you don't return in case of an error, put this into the else{} part
            while( q.next() )
            {
                if (q.value(0).toInt()>0)
                {
                    printf("\nThis personel ID was repeated\n");
                    break;
                }
                else
                {
                    q.prepare("SELECT CardID FROM Personel WHERE CardID =?;");
                    q.bindValue(0,card.c_str());
                    q.exec();

                    while(q.next())
                    {
                        printf("\nThis card is not valid : it was repeated\n");
                        break;
                    }
                }
            }

            q.prepare("insert into Personel value(?,?,?);");
            q.bindValue(0,id);
            q.bindValue(1,card.c_str());
            q.bindValue(2,name.c_str());
            q.exec();
            printf("\nInsert Personel complete\n");
            db.close();
            return;

        }
        else
        {
            string card;
            card=cmnd;
            QString dt=QDateTime::currentDateTime().toString("yyyy-MM-dd HH:mm:ss");

            q.prepare("SELECT P_id FROM Personel WHERE CardID =?;");
            q.bindValue(0,card.c_str());

            if( !q.exec() )
            {
              printf("\nThe error occured from database418\n");
              ShowMainMenu();

            }
            string result=("\n    CARD NOT VALID =>repeated     %s\n");
            // Note: if you don't return in case of an error, put this into the else{} part
            while( q.next() )
            {
                int P_id = q.value(0).toInt();
                q.prepare("insert into Absent(P_id,DateTime,key1,key2,key3,key4) value(?,?,1,0,0,0);");
                q.bindValue(0,P_id);
                q.bindValue(1,dt);
                q.exec();
                result=("\n********WELL COME **********%s\n");
            }
            printf("%s",result);
            db.close();
            return;
        }

    }
}

这是我的输出：

    0 - Press 0 to Back
    1 - Press 1 to Register the card
    1
    //the below lines when I want to select something from both tables , occurred
    No such file or directory
    QSqlError("", "Unable to fetch row", "No query")

    No such file or directory  
    //the below line is when the if(cmnd="Register") {..} block is running , occurred.
    QSqlDatabasePrivate::addDatabase: duplicate connection name 'qt_sql_default_connection', old connection removed.

Answer 1

每次SQL（）函数运行时，您都不需要关闭和打开数据库连接。更好的方法是在应用程序启动时打开db一次，最后关闭它。然后，在SQL（..）中，您只需创建类似

的查询

QSqlQuery q;

并与之合作。

QSqlError("", "Unable to fetch row", "No query")的原因是您在打开db之前创建QSqlQuery(db);。你还需要检查q.prepare（）是否有错误。

Answer 2

它解决了。做这些：
1 - 正如@Tony所说：QSqlQuery应在打开数据库后定义 2 - q.prepare仅用于SQL查询，包含bindvalue(),或select()等insert()。

创建sqlite表是行不通的

2 个答案: