Select data from database SQLite error - Help please!

Good afternoon.

How do I do a SELECT to an SQLite database?

I have the following:

Code:

---

QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
 
    db.setDatabaseName("bd_agt");
 
    if(db.open()){
        qDebug() << "Opening database, status: " << db.open();
    }
 
    QSqlQuery query("SELECT * FROM tag");
         while (query.next()) {
             QString nome = query.value(1).toString();
             ui->label->setText(nome);
         }
 
         qDebug() << query.size();

---

But in query.size () always returns -1, but I have values ​​in the database
How can I fix this?

Regards,
Daniel Sousa

check if the connection is opening.
Is your code getting into the if statement?

He says that the database opens. Where do I have to file for the database? Within the project folder or in the debug folder?


Regards,
Daniel Sousa

To see whether connection opens the statement at line 6 in your code should be executed.

It appears on the console: Opening database, status: true

if that is the case then everything seems ok,try show output of nome,and declare it out side while statement.

I've seen the problem! I put subestitui db.setDatabaseName by line 3 ("/ Users / danielsousa / Qt / BaseDados / bd_agt");

However I want to use only the name of the database that is to take on multiple operating systems. How do I do that?


regards,
Daniel Sousa

Try this code:

Code:

---

QSqlDatabase db = QSqlDatabase::addDatabase( "QSQLITE", "bd_agt" );
db.setDatabaseName( "full_path_to_db" );
db.setUserName( "UserName" );
db.setPassword( "UserPass" );
 
if (db.open())
{
  QSqlQuery query( QSqlDatabase::database( "bd_agt" ) );
 
  if (query.exec( "SELECT * FROM tag" ))
  {
    while (query.next()) 
    {
      QString nome = query.value(1).toString();
      ui->label->setText( nome );
    }
 
    query.clear();
  }
  else
    qDebug() << "Error: " << query.lastError().text();
}
else
  qDebug() << "Error: " << db.lastError().databaseText();

---

ideally, the db file should be stored relative to your executable file. see QCoreApplication::applicationDirPath

qDebug() << query.size() == -1 ? query.numRowsAffected() : query.size();

Quote:

---

From documentation: QSqlQuery::size():
Returns the size of the result (number of rows returned), or -1 if the size cannot be determined or if the database does not support reporting information about query sizes.

---

Quote:

---

From documentation: QSqlQuery::numRowsAffected():
Returns the number of rows affected by the result's SQL statement, or -1 if it cannot be determined.

---

As per quotes above, if everything works but size is -1, then it can't be determined and that's all.

When working with SQLite, numRowsAffected() nor size() never worked for me.

As long as next() works, you have nothing to worry about.

Hello
I'm trying to find a record in a database through the name. I have the following code:

Code:

---

//Metodo que procura uma tag através do seu nome
bool BaseDados::procurarTagPeloNome(QString nome){
    QSqlQuery query;
    query.prepare("SELECT * FROM tag WHERE nome=:nome ");
    query.bindValue(":nome", nome);
    query.exec();
 
    qDebug() << query.size();
 
    if(!query.isSelect()){
        return true;
    }else{
        return false;
    }
}

---

However, the size of me and I have always -1 there a record with the same name.
Am I doing something wrong?

Regards,
Daniel Sousa

Please read the post immediately above yours. Only some database engines support reporting the number of rows returned in a query. For Sqlite:

Code:

---

QSqlDatabase db;
    qDebug() << db.driver()->hasFeature(QSqlDriver::QuerySize);
// Output
false

---

If you only want to know if a record exists then:

Code:

---

//Metodo que procura uma tag através do seu nome
bool BaseDados::procurarTagPeloNome(const QString &nome){
    QSqlQuery query;
    query.prepare("SELECT 1 FROM tag WHERE nome=:nome ");
    query.bindValue(":nome", nome);
    return query.exec() && qry.next();
// If the query succeeds and returns a row (or more) then returns true.
}

---

Quote:

---

Originally Posted by ChrisW67

Please read the post immediately above yours. Only some database engines support reporting the number of rows returned in a query. For Sqlite:

Code:

---

QSqlDatabase db;
    qDebug() << db.driver()->hasFeature(QSqlDriver::QuerySize);
// Output
false

---

If you only want to know if a record exists then:

Code:

---

//Metodo que procura uma tag através do seu nome
bool BaseDados::procurarTagPeloNome(const QString &nome){
    QSqlQuery query;
    query.prepare("SELECT 1 FROM tag WHERE nome=:nome ");
    query.bindValue(":nome", nome);
    return query.exec() && qry.next();
// If the query succeeds and returns a row (or more) then returns true.
}

---

---

Thank longer works. I thought only query.size () is that eventually could not be supported.