Sqlite connection weird thing

ChrisW67

IIRC Sqlite does not actually create the file at open() but delays until something is written to it. If the file location is in an unwritable location it will fail at the time of the first CREATE TABLE etc. You are using a relative path to the database file. The file will go into the current working directory of the process at that time, which may not be writeable. This is especially the case with Windows when an executable is installed into Program Files. Often that directory is the default working directory and users cannot write there.

Using an absolute path to a writable location will fix this.

masa4

@ChrisW67 Thanks for reply, i am using linux but i switched to absolute path.

Emre MUTLU

Can you show your code in a little more detail?

jsulm

@masa4 said in Sqlite connection weird thing:

Why this happens any idea?

Please add error handling: https://doc.qt.io/qt-6/qsqldatabase.html#lastError and https://doc.qt.io/qt-6/qsqlquery.html#lastError

masa4

@Emre-MUTLU
db.cpp:

DB::DB(const QString &path)
{
    m_db = QSqlDatabase::addDatabase("QSQLITE");
    m_db.setDatabaseName(path);

    if(!m_db.open())
        qDebug() << "Error: connection with database failed";
    else
        qDebug() << "Database: connection ok";
}

Its the constructor. im passing path as default value in header: db.h:

//DB(const QString& path = "/home/projectfolder/mydbname"); //this one works
DB(const QString& path = "mydbname"); //but i need this one

jsulm

@masa4 I repeat myself: Please add error handling: https://doc.qt.io/qt-6/qsqldatabase.html#lastError and https://doc.qt.io/qt-6/qsqlquery.html#lastError

masa4

@jsulm Thank you for advice. I added it like:

DB::DB(const QString &path)
{
    m_db = QSqlDatabase::addDatabase("QSQLITE");
    m_db.setDatabaseName(path);

    if(!m_db.open())
        qDebug() << "Error: connection with database failed";
    else
        qDebug() << "Database: connection ok";
    qDebug() << m_db.lastError().text();
}

When i used in this way and set the path's value to a wrong value it returns empty text. no error. and output is "Database: connection ok" but actually its not ok.

jsulm

@masa4 You wrote before: "my sql queries does not run". So, please add error handling in the code where you're executing SQL queries to see what exactly the problem is (https://doc.qt.io/qt-6/qsqlquery.html#lastError).

Emre MUTLU

maybe your quaries not work right cause your code work for me

masa4

@jsulm
one function in db.cpp:

double DB::selectData()
{
    QSqlQuery query;
    query.prepare("SELECT col1 FROM mytable");

    if(query.exec()){
        query.first();
        return query.value(0).toDouble();
    }
    else{
        query.lastError().text();
        return -1;
    }
}

mainwidget.h:

public:
DB *db = new DB;

mainwidget.cpp:

ui->labeldb->setText(QString::number(db->selectData()));

When path value is setted to like "mydbname" the return value -1 show up on my label. If i corrected path with absolute value like "/home/projectfolder/mydbname" i got right value on my label(its 10)
But still no error messages. Error messages show as empty text, like this: ""

jsulm

@masa4 said in Sqlite connection weird thing:

But still no error messages

Of course not - you are not printing the error message...

qDebug() << query.lastError().text();

Emre MUTLU

double DB::selectData()
{
    QSqlQuery query;
    query.prepare("SELECT col1 FROM mytable");

    if(query.exec()){
       while(query.first()) {
        return query.value(0).toDouble();
      }
    }
    else{
        qDebug()<<query.lastError();
        return -1;
    }
}

try this

masa4

@jsulm ow sorry didnt realize it. its:

"No query Unable to fetch row"

masa4

@Emre-MUTLU said in Sqlite connection weird thing:

if(query.exec()){
while(query.first()) {
return query.value(0).toDouble();
}
}

nope. same error. But actual problem is this code works when i use absolute path. But not for relative path.

Emre MUTLU

did you check db is open when you trying to using query

Emre MUTLU

double DB::selectData()
{
  if(db.open()) {
    QSqlQuery query;
    query.prepare("SELECT col1 FROM mytable");

    if(query.exec()){
       while(query.first()) {
        return query.value(0).toDouble();
      }
    } else{
        qDebug()<<query.lastError();
        return -1;
    }
}else {
qDebug()<<db.lastError();
}
}

something like this

jsulm

@Emre-MUTLU You should also check the return value of query.prepare("SELECT col1 FROM mytable");
Try also:

QSqlQuery query("SELECT col1 FROM mytable");

Emre MUTLU

@jsulm

double DB::selectData()
{
  if(db.open()) {
    QSqlQuery query("SELECT col1 FROM mytable");
    if(query.exec()){
       while(query.first()) {
        return query.value(0).toDouble();
      }
    } else{
        qDebug()<<query.lastError();
        return -1;
    }
}else {
qDebug()<<db.lastError();
}
}

masa4

@jsulm said in Sqlite connection weird thing:

isopen returns true. query.prepare() returns false.

jsulm

@Emre-MUTLU said in Sqlite connection weird thing:

double DB::selectData()
{
if(db.open()) {
QSqlQuery query("SELECT col1 FROM mytable");
if(query.exec()){
while(query.first()) {
return query.value(0).toDouble();
}
} else{
qDebug()<<query.lastError();
return -1;
}
}else {
qDebug()<<db.lastError();
}
}

You forgot to mention whether this version works...